diff --git a/irc.go b/irc.go
index 179a089..4b5f4d3 100644
--- a/irc.go
+++ b/irc.go
@@ -2,6 +2,7 @@ package soju
 
 import (
 	"fmt"
+	"sort"
 	"strings"
 
 	"gopkg.in/irc.v3"
@@ -18,6 +19,9 @@ const (
 
 const maxMessageLength = 512
 
+// The server-time layout, as defined in the IRCv3 spec.
+const serverTimeLayout = "2006-01-02T15:04:05.000Z"
+
 type userModes string
 
 func (ms userModes) Has(c byte) bool {
@@ -293,5 +297,75 @@ type batch struct {
 	Label  string
 }
 
-// The server-time layout, as defined in the IRCv3 spec.
-const serverTimeLayout = "2006-01-02T15:04:05.000Z"
+func join(channels, keys []string) []*irc.Message {
+	// Put channels with a key first
+	js := joinSorter{channels, keys}
+	sort.Sort(&js)
+
+	// Two spaces because there are three words (JOIN, channels and keys)
+	maxLength := maxMessageLength - (len("JOIN") + 2)
+
+	var msgs []*irc.Message
+	var channelsBuf, keysBuf strings.Builder
+	for i, channel := range channels {
+		key := keys[i]
+
+		n := channelsBuf.Len() + keysBuf.Len() + 1 + len(channel)
+		if key != "" {
+			n += 1 + len(key)
+		}
+
+		if channelsBuf.Len() > 0 && n > maxLength {
+			// No room for the new channel in this message
+			params := []string{channelsBuf.String()}
+			if keysBuf.Len() > 0 {
+				params = append(params, keysBuf.String())
+			}
+			msgs = append(msgs, &irc.Message{Command: "JOIN", Params: params})
+			channelsBuf.Reset()
+			keysBuf.Reset()
+		}
+
+		if channelsBuf.Len() > 0 {
+			channelsBuf.WriteByte(',')
+		}
+		channelsBuf.WriteString(channel)
+		if key != "" {
+			if keysBuf.Len() > 0 {
+				keysBuf.WriteByte(',')
+			}
+			keysBuf.WriteString(key)
+		}
+	}
+	if channelsBuf.Len() > 0 {
+		params := []string{channelsBuf.String()}
+		if keysBuf.Len() > 0 {
+			params = append(params, keysBuf.String())
+		}
+		msgs = append(msgs, &irc.Message{Command: "JOIN", Params: params})
+	}
+
+	return msgs
+}
+
+type joinSorter struct {
+	channels []string
+	keys     []string
+}
+
+func (js *joinSorter) Len() int {
+	return len(js.channels)
+}
+
+func (js *joinSorter) Less(i, j int) bool {
+	if (js.keys[i] != "") != (js.keys[j] != "") {
+		// Only one of the channels has a key
+		return js.keys[i] != ""
+	}
+	return js.channels[i] < js.channels[j]
+}
+
+func (js *joinSorter) Swap(i, j int) {
+	js.channels[i], js.channels[j] = js.channels[j], js.channels[i]
+	js.keys[i], js.keys[j] = js.keys[j], js.keys[i]
+}
diff --git a/upstream.go b/upstream.go
index 22b63fe..76479a8 100644
--- a/upstream.go
+++ b/upstream.go
@@ -553,19 +553,15 @@ func (uc *upstreamConn) handleMessage(msg *irc.Message) error {
 		})
 
 		if len(uc.network.channels) > 0 {
-			// TODO: split this into multiple messages if need be
-			var names, keys []string
+			var channels, keys []string
 			for _, ch := range uc.network.channels {
-				names = append(names, ch.Name)
+				channels = append(channels, ch.Name)
 				keys = append(keys, ch.Key)
 			}
-			uc.SendMessage(&irc.Message{
-				Command: "JOIN",
-				Params: []string{
-					strings.Join(names, ","),
-					strings.Join(keys, ","),
-				},
-			})
+
+			for _, msg := range join(channels, keys) {
+				uc.SendMessage(msg)
+			}
 		}
 	case irc.RPL_MYINFO:
 		if err := parseMessageParams(msg, nil, &uc.serverName, nil, &uc.availableUserModes, nil); err != nil {