diff --git a/src/persist-client/src/cache.rs b/src/persist-client/src/cache.rs
index 2fcb803ef0db9..f70b991eb54bb 100644
--- a/src/persist-client/src/cache.rs
+++ b/src/persist-client/src/cache.rs
@@ -33,6 +33,7 @@ use tracing::{debug, instrument};
 
 use crate::async_runtime::IsolatedRuntime;
 use crate::error::{CodecConcreteType, CodecMismatch};
+use crate::internal::cache::BlobMemCache;
 use crate::internal::machine::retry_external;
 use crate::internal::metrics::{LockMetrics, Metrics, MetricsBlob, MetricsConsensus, ShardMetrics};
 use crate::internal::state::TypedState;
@@ -218,6 +219,9 @@ impl PersistClientCache {
                     Self::PROMETHEUS_SCRAPE_INTERVAL,
                 )
                 .await;
+                // This is intentionally "outside" (wrapping) MetricsBlob so
+                // that we don't include cached responses in blob metrics.
+                let blob = BlobMemCache::new(&self.cfg, Arc::clone(&self.metrics), blob);
                 Arc::clone(&x.insert((RttLatencyTask(task.abort_on_drop()), blob)).1)
             }
         };
diff --git a/src/persist-client/src/internal/cache.rs b/src/persist-client/src/internal/cache.rs
new file mode 100644
index 0000000000000..a9def6ae82805
--- /dev/null
+++ b/src/persist-client/src/internal/cache.rs
@@ -0,0 +1,390 @@
+// Copyright Materialize, Inc. and contributors. All rights reserved.
+//
+// Use of this software is governed by the Business Source License
+// included in the LICENSE file.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0.
+
+//! In-process caches of [Blob].
+
+use std::sync::{Arc, Mutex};
+
+use async_trait::async_trait;
+use bytes::Bytes;
+use mz_ore::bytes::SegmentedBytes;
+use mz_ore::cast::CastFrom;
+use mz_persist::location::{Atomicity, Blob, BlobMetadata, ExternalError};
+
+use crate::cfg::PersistConfig;
+use crate::internal::metrics::Metrics;
+
+// In-memory cache for [Blob].
+#[derive(Debug)]
+pub struct BlobMemCache {
+    metrics: Arc<Metrics>,
+    cache: Mutex<lru::Lru<String, SegmentedBytes>>,
+    blob: Arc<dyn Blob + Send + Sync>,
+}
+
+impl BlobMemCache {
+    pub fn new(
+        cfg: &PersistConfig,
+        metrics: Arc<Metrics>,
+        blob: Arc<dyn Blob + Send + Sync>,
+    ) -> Arc<dyn Blob + Send + Sync> {
+        // WIP let eviction_metrics = Arc::clone(&metrics);
+        // TODO: Make this react dynamically to changes in configuration.
+        let cache = lru::Lru::new(cfg.dynamic.blob_cache_mem_limit_bytes());
+        let blob = BlobMemCache {
+            metrics,
+            cache: Mutex::new(cache),
+            blob,
+        };
+        Arc::new(blob)
+    }
+
+    fn update_size_metrics(&self) {
+        // WIP skip the double lock here
+        let cache = self.cache.lock().expect("WIP");
+        self.metrics
+            .blob_cache_mem
+            .size_blobs
+            .set(u64::cast_from(cache.entry_count()));
+        self.metrics
+            .blob_cache_mem
+            .size_bytes
+            .set(u64::cast_from(cache.weighted_size()));
+    }
+}
+
+#[async_trait]
+impl Blob for BlobMemCache {
+    async fn get(&self, key: &str) -> Result<Option<SegmentedBytes>, ExternalError> {
+        // First check if the blob is in the cache. If it is, return it. If not,
+        // fetch it and put it in the cache.
+        //
+        // Blobs are write-once modify-never, so we don't have to worry about
+        // any races or cache invalidations here. If the value is in the cache,
+        // it's also what's in s3 (if not, then there's a horrible bug somewhere
+        // else).
+        if let Some(cached_value) = self.cache.lock().expect("WIP").get(key) {
+            self.metrics.blob_cache_mem.hits_blobs.inc();
+            self.metrics
+                .blob_cache_mem
+                .hits_bytes
+                .inc_by(u64::cast_from(cached_value.len()));
+            return Ok(Some(cached_value.clone()));
+        }
+
+        let res = self.blob.get(key).await?;
+        if let Some(blob) = res.as_ref() {
+            self.cache
+                .lock()
+                .expect("WIP")
+                .insert(key.to_owned(), blob.clone(), blob.len());
+            self.update_size_metrics();
+        }
+        Ok(res)
+    }
+
+    async fn list_keys_and_metadata(
+        &self,
+        key_prefix: &str,
+        f: &mut (dyn FnMut(BlobMetadata) + Send + Sync),
+    ) -> Result<(), ExternalError> {
+        self.blob.list_keys_and_metadata(key_prefix, f).await
+    }
+
+    async fn set(&self, key: &str, value: Bytes, atomic: Atomicity) -> Result<(), ExternalError> {
+        let () = self.blob.set(key, value.clone(), atomic).await?;
+        let weight = value.len();
+        self.cache
+            .lock()
+            .expect("WIP")
+            .insert(key.to_owned(), SegmentedBytes::from(value), weight);
+        self.update_size_metrics();
+        Ok(())
+    }
+
+    async fn delete(&self, key: &str) -> Result<Option<usize>, ExternalError> {
+        let res = self.blob.delete(key).await;
+        self.cache.lock().expect("WIP").remove(key);
+        self.update_size_metrics();
+        res
+    }
+
+    async fn restore(&self, key: &str) -> Result<(), ExternalError> {
+        self.blob.restore(key).await
+    }
+}
+
+mod lru_list {
+    use mz_ore::collections::HashMap;
+
+    #[derive(Clone, Debug, Hash, PartialEq, Eq)]
+    pub struct NodeId(u64);
+
+    pub struct ListNode<T> {
+        next: Option<NodeId>,
+        prev: Option<NodeId>,
+
+        payload: T,
+    }
+
+    impl<T> std::fmt::Debug for ListNode<T> {
+        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            f.debug_struct("ListNode")
+                .field("next", &self.next)
+                .field("prev", &self.prev)
+                .finish_non_exhaustive()
+        }
+    }
+
+    pub struct List<T> {
+        next_id: NodeId,
+        head: Option<NodeId>,
+        tail: Option<NodeId>,
+        nodes: HashMap<NodeId, ListNode<T>>,
+    }
+
+    impl<T> std::fmt::Debug for List<T> {
+        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            f.debug_struct("List")
+                .field("next_id", &self.next_id)
+                .field("head", &self.head)
+                .field("tail", &self.tail)
+                .field("nodes", &self.nodes)
+                .finish()
+        }
+    }
+
+    impl<T> Default for List<T> {
+        fn default() -> Self {
+            Self {
+                next_id: NodeId(0),
+                nodes: HashMap::new(),
+                head: None,
+                tail: None,
+            }
+        }
+    }
+
+    impl<T> List<T> {
+        pub fn next_id(&mut self) -> NodeId {
+            let id = self.next_id.clone();
+            self.next_id.0 += 1;
+            id
+        }
+
+        pub fn peek_tail(&self) -> Option<(&NodeId, &T)> {
+            let id = self.tail.as_ref()?;
+            let node = self.nodes.get(id).expect("WIP");
+            Some((id, &node.payload))
+        }
+
+        pub fn push_head(&mut self, id: NodeId, payload: T) {
+            let next = match self.head.as_mut() {
+                Some(head) => {
+                    self.nodes.get_mut(head).expect("WIP").prev = Some(id.clone());
+                    let next = head.clone();
+                    *head = id.clone();
+                    Some(next)
+                }
+                None => {
+                    self.head = Some(id.clone());
+                    self.tail = Some(id.clone());
+                    None
+                }
+            };
+            let node = ListNode {
+                next,
+                prev: None,
+                payload,
+            };
+            self.nodes.insert(id.clone(), node);
+        }
+
+        pub fn get(&self, id: &NodeId) -> Option<&T> {
+            self.nodes.get(id).map(|x| &x.payload)
+        }
+
+        pub fn remove(&mut self, id: &NodeId) -> Option<T> {
+            let node = self.nodes.remove(id)?;
+            if let Some(next_id) = node.next.as_ref() {
+                self.nodes.get_mut(next_id).expect("WIP").prev = node.prev.clone();
+            } else {
+                // No next means this was the tail
+                assert_eq!(self.tail.as_ref(), Some(id));
+                self.tail = node.prev.clone();
+            }
+            if let Some(prev_id) = node.prev.as_ref() {
+                self.nodes.get_mut(prev_id).expect("WIP").next = node.next.clone();
+            } else {
+                // No prev means this was the head
+                assert_eq!(self.head.as_ref(), Some(id));
+                self.head = node.next.clone();
+            }
+            Some(node.payload)
+        }
+
+        pub fn iter(&self) -> impl Iterator<Item = (&NodeId, &T)> {
+            ListIter {
+                list: self,
+                next: self.head.as_ref(),
+            }
+            .map(|(id, node)| (id, &node.payload))
+        }
+
+        pub fn validate(&self) {
+            assert_eq!(self.nodes.is_empty(), self.head.is_none());
+            assert_eq!(self.nodes.is_empty(), self.tail.is_none());
+
+            let mut count = 0;
+            let mut expected_prev = None;
+            let mut iter = ListIter {
+                list: self,
+                next: self.head.as_ref(),
+            };
+            while let Some((id, node)) = iter.next() {
+                count += 1;
+                assert_eq!(node.prev.as_ref(), expected_prev);
+                expected_prev = Some(id);
+            }
+            assert_eq!(self.tail.as_ref(), expected_prev);
+            assert_eq!(count, self.nodes.len());
+        }
+    }
+
+    struct ListIter<'a, T> {
+        list: &'a List<T>,
+        next: Option<&'a NodeId>,
+    }
+
+    impl<'a, T> Iterator for ListIter<'a, T> {
+        type Item = (&'a NodeId, &'a ListNode<T>);
+
+        fn next(&mut self) -> Option<Self::Item> {
+            let id = self.next?;
+            let node = self.list.nodes.get(id).expect("WIP");
+            self.next = node.next.as_ref();
+            Some((id, node))
+        }
+    }
+}
+
+mod lru {
+    use std::borrow::Borrow;
+    use std::hash::Hash;
+
+    use mz_ore::collections::HashMap;
+
+    use crate::internal::cache::lru_list;
+
+    pub struct LruNode<K, V> {
+        key: K,
+        val: V,
+        weight: usize,
+    }
+
+    pub struct Lru<K, V> {
+        capacity: usize,
+        total_weight: usize,
+
+        nodes: lru_list::List<LruNode<K, V>>,
+        by_key: HashMap<K, lru_list::NodeId>,
+    }
+
+    impl<K: std::fmt::Debug, V> std::fmt::Debug for Lru<K, V> {
+        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            f.debug_struct("Lru")
+                .field("capacity", &self.capacity)
+                .field("total_weight", &self.total_weight)
+                // WIP .field("nodes", &self.nodes)
+                .field("by_key", &self.by_key)
+                .finish()
+        }
+    }
+
+    impl<K: Hash + Eq + Clone, V> Lru<K, V> {
+        pub fn new(capacity: usize) -> Self {
+            Lru {
+                capacity,
+                total_weight: 0,
+                nodes: lru_list::List::default(),
+                by_key: HashMap::new(),
+            }
+        }
+
+        pub fn get<Q>(&mut self, key: &Q) -> Option<&V>
+        where
+            K: Borrow<Q>,
+            Q: Hash + Eq + ?Sized,
+        {
+            let id = self.by_key.get(key)?;
+            let payload = self.nodes.remove(id).expect("WIP");
+            self.nodes.push_head(id.clone(), payload);
+            self.validate();
+            let payload = self.nodes.get(&id).expect("WIP");
+            Some(&payload.val)
+        }
+
+        pub fn insert(&mut self, key: K, val: V, weight: usize) {
+            let id = self.nodes.next_id();
+            let payload = LruNode { key, val, weight };
+
+            let _ = self.remove(&payload.key);
+            self.total_weight += weight;
+            self.by_key.insert(payload.key.clone(), id.clone());
+            self.nodes.push_head(id, payload);
+            self.resize();
+            self.validate();
+        }
+
+        pub fn entry_count(&self) -> usize {
+            self.by_key.len()
+        }
+
+        pub fn weighted_size(&self) -> usize {
+            self.total_weight
+        }
+
+        pub fn remove<Q>(&mut self, k: &Q) -> Option<LruNode<K, V>>
+        where
+            K: Borrow<Q>,
+            Q: Hash + Eq + ?Sized,
+        {
+            let id = self.by_key.remove(k)?;
+            let ret = self.remove_id(id);
+            self.validate();
+            ret
+        }
+
+        fn remove_id(&mut self, id: lru_list::NodeId) -> Option<LruNode<K, V>> {
+            let payload = self.nodes.remove(&id)?;
+            self.total_weight -= payload.weight;
+            Some(payload)
+        }
+
+        fn resize(&mut self) {
+            while self.total_weight > self.capacity {
+                let (id, node) = self.nodes.peek_tail().expect("WIP");
+                self.by_key.remove(&node.key);
+                self.remove_id(id.clone());
+            }
+        }
+
+        fn validate(&self) {
+            self.nodes.validate();
+            assert!(self.total_weight <= self.capacity);
+
+            let mut w = 0;
+            for (id, payload) in self.nodes.iter() {
+                w += payload.weight;
+                assert_eq!(self.by_key.get(&payload.key), Some(id));
+            }
+            assert_eq!(w, self.total_weight);
+        }
+    }
+}
diff --git a/src/persist-client/src/internal/metrics.rs b/src/persist-client/src/internal/metrics.rs
index a995dfc9089bf..05c0e8e24e11c 100644
--- a/src/persist-client/src/internal/metrics.rs
+++ b/src/persist-client/src/internal/metrics.rs
@@ -2261,12 +2261,12 @@ impl ConsolidationMetrics {
 }
 
 #[derive(Debug)]
-#[allow(dead_code)] // TODO: Remove this when we reintroduce the cache.
 pub struct BlobMemCache {
     pub(crate) size_blobs: UIntGauge,
     pub(crate) size_bytes: UIntGauge,
     pub(crate) hits_blobs: IntCounter,
     pub(crate) hits_bytes: IntCounter,
+    #[allow(dead_code)]
     pub(crate) evictions: IntCounter,
 }
 
diff --git a/src/persist-client/src/lib.rs b/src/persist-client/src/lib.rs
index e09157a59d5e4..ca6e129c954f2 100644
--- a/src/persist-client/src/lib.rs
+++ b/src/persist-client/src/lib.rs
@@ -153,6 +153,7 @@ pub mod write;
 /// An implementation of the public crate interface.
 mod internal {
     pub mod apply;
+    pub mod cache;
     pub mod compact;
     pub mod encoding;
     pub mod gc;