diff --git a/src/locale/br.js b/src/locale/br.js
index bbd242c7..17e4ee50 100644
--- a/src/locale/br.js
+++ b/src/locale/br.js
@@ -1,6 +1,47 @@
 // Breton [br]
 import dayjs from 'dayjs'
 
+function lastNumber(number) {
+  if (number > 9) {
+    return lastNumber(number % 10)
+  }
+  return number
+}
+function softMutation(text) {
+  const mutationTable = {
+    m: 'v',
+    b: 'v',
+    d: 'z'
+  }
+  return mutationTable[text.charAt(0)] + text.substring(1)
+}
+function mutation(text, number) {
+  if (number === 2) {
+    return softMutation(text)
+  }
+  return text
+}
+function relativeTimeWithMutation(number, withoutSuffix, key) {
+  const format = {
+    mm: 'munutenn',
+    MM: 'miz',
+    dd: 'devezh'
+  }
+  return `${number} ${mutation(format[key], number)}`
+}
+function specialMutationForYears(number) {
+  switch (lastNumber(number)) {
+    case 1:
+    case 3:
+    case 4:
+    case 5:
+    case 9:
+      return `${number} bloaz`
+    default:
+      return `${number} vloaz`
+  }
+}
+
 const locale = {
   name: 'br',
   weekdays: 'Sul_Lun_Meurzh_Mercʼher_Yaou_Gwener_Sadorn'.split('_'),
@@ -18,6 +59,21 @@ const locale = {
     LLL: 'D [a viz] MMMM YYYY h[e]mm A',
     LLLL: 'dddd, D [a viz] MMMM YYYY h[e]mm A'
   },
+  relativeTime: {
+    future: 'a-benn %s',
+    past: '%s ʼzo',
+    s: 'un nebeud segondennoù',
+    m: 'ur vunutenn',
+    mm: relativeTimeWithMutation,
+    h: 'un eur',
+    hh: '%d eur',
+    d: 'un devezh',
+    dd: relativeTimeWithMutation,
+    M: 'ur miz',
+    MM: relativeTimeWithMutation,
+    y: 'ur bloaz',
+    yy: specialMutationForYears
+  },
   meridiem: hour => (hour < 12 ? 'a.m.' : 'g.m.') // a-raok merenn | goude merenn
 }
 
diff --git a/test/locale/br.test.js b/test/locale/br.test.js
new file mode 100644
index 00000000..1aa106a4
--- /dev/null
+++ b/test/locale/br.test.js
@@ -0,0 +1,54 @@
+import moment from 'moment'
+import MockDate from 'mockdate'
+import dayjs from '../../src'
+import relativeTime from '../../src/plugin/relativeTime'
+import '../../src/locale/br'
+
+dayjs.extend(relativeTime)
+
+beforeEach(() => {
+  MockDate.set(new Date())
+})
+
+afterEach(() => {
+  MockDate.reset()
+})
+
+it('Format Month with locale function', () => {
+  for (let i = 0; i <= 7; i += 1) {
+    const dayjsBR = dayjs().locale('br').add(i, 'day')
+    const momentBR = moment().locale('br').add(i, 'day')
+    const testFormat1 = 'DD MMMM YYYY MMM'
+    const testFormat2 = 'MMMM'
+    const testFormat3 = 'MMM'
+    expect(dayjsBR.format(testFormat1)).toEqual(momentBR.format(testFormat1))
+    expect(dayjsBR.format(testFormat2)).toEqual(momentBR.format(testFormat2))
+    expect(dayjsBR.format(testFormat3)).toEqual(momentBR.format(testFormat3))
+  }
+})
+
+it('RelativeTime: Time from X', () => {
+  const T = [
+    [44.4, 'second'], // a few seconds
+    [89.5, 'second'], // a minute
+    [130, 'second'], // two minutes
+    [43, 'minute'], // 44 minutes
+    [1, 'hour'], // 1 hour
+    [21, 'hour'], // 21 hours
+    [2, 'day'], // 2 days
+    [25, 'day'], // 25 days
+    [2, 'month'], // 2 months
+    [10, 'month'], // 10 months
+    [18, 'month'], // 2 years
+    [15, 'year'] // 15 years
+  ]
+
+  T.forEach((t) => {
+    dayjs.locale('br')
+    moment.locale('br')
+    expect(dayjs().from(dayjs().add(t[0], t[1])))
+      .toBe(moment().from(moment().add(t[0], t[1])))
+    expect(dayjs().from(dayjs().add(t[0], t[1]), true))
+      .toBe(moment().from(moment().add(t[0], t[1]), true))
+  })
+})