diff --git a/CHANGELOG_PENDING.md b/CHANGELOG_PENDING.md
index d0980f1d9..5c23bd7b4 100644
--- a/CHANGELOG_PENDING.md
+++ b/CHANGELOG_PENDING.md
@@ -23,6 +23,7 @@ Friendly reminder, we have a [bug bounty program](https://hackerone.com/tendermi
 - Apps
   - [ABCI] \#5447 Remove `SetOption` method from `ABCI.Client` interface
   - [ABCI] \#5447 Reset `Oneof` indexes for  `Request` and `Response`.
+  - [ABCI] \#5818 Use protoio for msg length delimitation. Migrates from int64 to uint64 length delimiters.
 
 - P2P Protocol
 
diff --git a/UPGRADING.md b/UPGRADING.md
index a6ce485d2..318928fe4 100644
--- a/UPGRADING.md
+++ b/UPGRADING.md
@@ -10,6 +10,8 @@ This guide provides instructions for upgrading to specific versions of Tendermin
 
 * The method `SetOption` has been removed from the ABCI.Client interface. This feature was used in the early ABCI implementation's.
 
+* Messages are written to a byte stream using uin64 length delimiters instead of int64.
+
 ### Config Changes
 
 * `fast_sync = "v1"` is no longer supported. Please use `v2` instead.
diff --git a/abci/types/messages.go b/abci/types/messages.go
index eaf1721dd..a80663646 100644
--- a/abci/types/messages.go
+++ b/abci/types/messages.go
@@ -1,11 +1,10 @@
 package types
 
 import (
-	"bufio"
-	"encoding/binary"
 	"io"
 
 	"github.com/gogo/protobuf/proto"
+	"github.com/tendermint/tendermint/libs/protoio"
 )
 
 const (
@@ -14,57 +13,19 @@ const (
 
 // WriteMessage writes a varint length-delimited protobuf message.
 func WriteMessage(msg proto.Message, w io.Writer) error {
-	bz, err := proto.Marshal(msg)
+	protoWriter := protoio.NewDelimitedWriter(w)
+	_, err := protoWriter.WriteMsg(msg)
 	if err != nil {
 		return err
 	}
-	return encodeByteSlice(w, bz)
+
+	return nil
 }
 
 // ReadMessage reads a varint length-delimited protobuf message.
 func ReadMessage(r io.Reader, msg proto.Message) error {
-	return readProtoMsg(r, msg, maxMsgSize)
-}
-
-func readProtoMsg(r io.Reader, msg proto.Message, maxSize int) error {
-	// binary.ReadVarint takes an io.ByteReader, eg. a bufio.Reader
-	reader, ok := r.(*bufio.Reader)
-	if !ok {
-		reader = bufio.NewReader(r)
-	}
-	length64, err := binary.ReadVarint(reader)
-	if err != nil {
-		return err
-	}
-	length := int(length64)
-	if length < 0 || length > maxSize {
-		return io.ErrShortBuffer
-	}
-	buf := make([]byte, length)
-	if _, err := io.ReadFull(reader, buf); err != nil {
-		return err
-	}
-	return proto.Unmarshal(buf, msg)
-}
-
-//-----------------------------------------------------------------------
-// NOTE: we copied wire.EncodeByteSlice from go-wire rather than keep
-// go-wire as a dep
-
-func encodeByteSlice(w io.Writer, bz []byte) (err error) {
-	err = encodeVarint(w, int64(len(bz)))
-	if err != nil {
-		return
-	}
-	_, err = w.Write(bz)
-	return
-}
-
-func encodeVarint(w io.Writer, i int64) (err error) {
-	var buf [10]byte
-	n := binary.PutVarint(buf[:], i)
-	_, err = w.Write(buf[0:n])
-	return
+	reader := protoio.NewDelimitedReader(r, maxMsgSize)
+	return reader.ReadMsg(msg)
 }
 
 //----------------------------------------