diff --git a/modules/core/src/main/scala/mess/codec/Decoder.scala b/modules/core/src/main/scala/mess/codec/Decoder.scala
index 9585f3a..3ac68c5 100644
--- a/modules/core/src/main/scala/mess/codec/Decoder.scala
+++ b/modules/core/src/main/scala/mess/codec/Decoder.scala
@@ -7,6 +7,8 @@ import mess.Fmt
 import mess.internal.ScalaVersionSpecifics._
 
 import scala.annotation.tailrec
+import scala.collection.Seq
+import scala.collection.immutable
 import scala.collection.mutable
 
 trait Decoder[A] extends Serializable { self =>
@@ -128,7 +130,7 @@ private[codec] trait Decoder1 {
           case Left(e)  => Left(e)
         }
 
-  @inline private[this] def decodeContainer[C[_], A](implicit
+  @inline private def decodeContainer[C[_], A](implicit
     decodeA: Decoder[A],
     factoryA: Factory[A, C[A]]
   ): Decoder[C[A]] =
@@ -150,7 +152,8 @@ private[codec] trait Decoder1 {
         }
     }
 
-  implicit def decodeSeq[A: Decoder]: Decoder[Seq[A]] = decodeContainer[Seq, A]
+  implicit def decodeSeq[A: Decoder]: Decoder[Seq[A]]            = decodeContainer[Seq, A]
+  implicit def decodeISeq[A: Decoder]: Decoder[immutable.Seq[A]] = decodeContainer[immutable.Seq, A]
 
   implicit def decodeSet[A: Decoder]: Decoder[Set[A]] = decodeContainer[Set, A]
 
diff --git a/modules/core/src/main/scala/mess/codec/Encoder.scala b/modules/core/src/main/scala/mess/codec/Encoder.scala
index 8f68602..ed6b4b6 100644
--- a/modules/core/src/main/scala/mess/codec/Encoder.scala
+++ b/modules/core/src/main/scala/mess/codec/Encoder.scala
@@ -5,6 +5,8 @@ import java.time.Instant
 import mess.Fmt
 
 import scala.annotation.tailrec
+import scala.collection.Seq
+import scala.collection.immutable
 import scala.collection.mutable
 
 trait Encoder[A] extends Serializable { self =>
@@ -74,6 +76,9 @@ private[codec] trait Encoder1 {
   implicit final def encodeSeq[A: Encoder]: Encoder.AsArray[Seq[A]] =
     a => Fmt.MArray(buildVector(a.iterator, Vector.newBuilder))
 
+  implicit final def encodeISeq[A: Encoder]: Encoder.AsArray[immutable.Seq[A]] =
+    a => Fmt.MArray(buildVector(a.iterator, Vector.newBuilder))
+
   implicit final def encodeSet[A: Encoder]: Encoder.AsArray[Set[A]] =
     a => Fmt.MArray(buildVector(a.iterator, Vector.newBuilder))
 
diff --git a/modules/core/src/test/scala/mess/codec/DecoderSpec.scala b/modules/core/src/test/scala/mess/codec/DecoderSpec.scala
index fc185a8..816e793 100644
--- a/modules/core/src/test/scala/mess/codec/DecoderSpec.scala
+++ b/modules/core/src/test/scala/mess/codec/DecoderSpec.scala
@@ -4,6 +4,9 @@ import mess.Fmt
 import mess.MsgpackHelper
 import mess.codec.semiauto._
 
+import scala.collection.Seq
+import scala.collection.immutable
+
 class DecoderSpec extends MsgpackHelper {
   case class Bar(double: Double)
 
@@ -53,7 +56,7 @@ class DecoderSpec extends MsgpackHelper {
   test("Decoder[Array[Byte]]") {
     val m  = Fmt.fromBytes(Array(0x11, 0x1a).map(_.toByte))
     val xs = decode[Array[Byte]](m).toTry.get
-    assertEquals(xs.toSeq, Seq(0x11, 0x1a).map(_.toByte))
+    assertEquals(xs.toSeq, immutable.Seq(0x11, 0x1a).map(_.toByte))
   }
   test("Decoder[Char] failure") {
     val msg = Fmt.fromString("ab")
@@ -188,6 +191,16 @@ class DecoderSpec extends MsgpackHelper {
     val msg = Fmt.fromString("")
     assertEquals(decode[Seq[Int]](msg), Left(TypeMismatchError("C[A]", msg)))
   }
+  check("Decoder[immutable.Seq[A]]",
+        Seq(
+          (immutable.Seq(0 to 14: _*), Fmt.fromValues((0 to 14).map(Fmt.fromInt): _*)),
+          (immutable.Seq.empty[Int], Fmt.unit)
+        )
+  )
+  test("Decoder[immutable.Seq[A]] failure") {
+    val msg = Fmt.fromString("")
+    assertEquals(decode[immutable.Seq[Int]](msg), Left(TypeMismatchError("C[A]", msg)))
+  }
   check("Decoder[Tuple2[A]]",
         Seq(
           ((1, 2), Fmt.fromValues(Fmt.fromByte(1.toByte), Fmt.fromByte(2.toByte)))
diff --git a/modules/core/src/test/scala/mess/codec/EncoderSpec.scala b/modules/core/src/test/scala/mess/codec/EncoderSpec.scala
index 4c214c8..2c1be74 100644
--- a/modules/core/src/test/scala/mess/codec/EncoderSpec.scala
+++ b/modules/core/src/test/scala/mess/codec/EncoderSpec.scala
@@ -4,6 +4,9 @@ import mess.Fmt
 import mess.MsgpackHelper
 import org.msgpack.core.MessagePack
 
+import scala.collection.Seq
+import scala.collection.immutable
+
 class EncoderSpec extends MsgpackHelper {
   case class Bar(double: Double)
 
@@ -123,6 +126,13 @@ class EncoderSpec extends MsgpackHelper {
           (Seq.empty[Int], Fmt.fromValues())
         )
   )
+  check(
+    "Encoder[immutable.Seq[A]]",
+    Seq(
+      (immutable.Seq(0 to 14: _*), Fmt.fromValues((0 to 14).map(a => Fmt.fromByte(a.toByte)): _*)),
+      (immutable.Seq.empty[Int], Fmt.fromValues())
+    )
+  )
   check("Encoder[List[A]]",
         Seq(
           ((0 to 14).toList, Fmt.fromValues((0 to 14).map(a => Fmt.fromByte(a.toByte)): _*))