diff --git a/CHANGELOG.md b/CHANGELOG.md
index ba2866023..55ef74a42 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -232,6 +232,8 @@
   were made to be indeed lazy by the effect, but their requirements were strengthened
   from `Applicative` to `Monad`.
 
+* Implements `Sized` for `Data.Seq.Sized` and `Data.Seq.Unsized`.
+
 #### Papers
 
 * In `Control.DivideAndConquer`: a port of the paper
diff --git a/libs/contrib/Data/Seq/Sized.idr b/libs/contrib/Data/Seq/Sized.idr
index 748363844..43578cbd7 100644
--- a/libs/contrib/Data/Seq/Sized.idr
+++ b/libs/contrib/Data/Seq/Sized.idr
@@ -207,3 +207,7 @@ public export
 implementation {n : Nat} -> Applicative (Seq n) where
   pure = replicate n
   (<*>) = zipWith ($)
+
+public export
+implementation Sized (Seq n a) where
+  size (MkSeq s) = size s
diff --git a/libs/contrib/Data/Seq/Unsized.idr b/libs/contrib/Data/Seq/Unsized.idr
index b4b850fd7..cb0eb28f5 100644
--- a/libs/contrib/Data/Seq/Unsized.idr
+++ b/libs/contrib/Data/Seq/Unsized.idr
@@ -218,3 +218,7 @@ public export
 public export
 implementation Monad Seq where
   xs >>= f = foldMap f xs
+
+public export
+implementation Sized (Seq a) where
+  size (MkSeq s) = size s