diff --git a/src/github.com/matrix-org/dendrite/appservice/routing/user.go b/src/github.com/matrix-org/dendrite/appservice/routing/user.go
index d2e02e94e..4f9a8e863 100644
--- a/src/github.com/matrix-org/dendrite/appservice/routing/user.go
+++ b/src/github.com/matrix-org/dendrite/appservice/routing/user.go
@@ -3,9 +3,11 @@ package routing
 import (
 	"github.com/matrix-org/util"
 	"net/http"
+	"strings"
+	"unicode"
 )
 
-// represents response to an AppService URI to User Id
+// URIToUIDResponse represents response to an AppService URI to User Id
 // (/_matrix/app/r0/user?uri={url_encoded_uri}) request
 type URIToUIDResponse struct {
 	UserID string `json:"user_id"`
@@ -15,9 +17,9 @@ type URIToUIDResponse struct {
 // enables users to contact appservice users directly by taking an encoded
 // URI and turning it into a Matrix ID on the homeserver.
 // https://matrix.org/docs/spec/application_service/unstable.html#user-ids
-
+// tel://123.1234 -> @tel_//123.1234:matrix.org
+// mailto:test@matrix.org -> @mailto_test_matrix.org:matrix.org
 func URIToUID(req *http.Request, cfg config.Dendrite) util.JSONResponse {
-	// TODO: Implement
 	homeserver := cfg.Matrix.ServerName
 	uri := req.URL.Query().Get("uri")
 	if uri == "" {
@@ -26,8 +28,25 @@ func URIToUID(req *http.Request, cfg config.Dendrite) util.JSONResponse {
 			JSON: nil,
 		}
 	}
+	// V1 just replaces the illegal characters (from
+	// https://matrix.org/docs/spec/appendices.html#id12) with _
+	// TODO: Come up with a better way to turn URIs into User IDs
+	w := strings.FieldsFunc(strings.ToLower(uri), func(r rune) bool {
+		if unicode.IsDigit(r) || unicode.IsLetter(r) {
+			return false
+		}
+		switch r {
+		case '.', '_', '=', '-', '/':
+			return false
+		}
+		return true
+	})
+
+	// compile user ID and return
+	userID := "@" + strings.Join(w, "_") + ":" + homeserver
+
 	return util.JSONResponse{
 		Code: http.StatusOK,
-		JSON: nil,
+		JSON: URIToUIDResponse{UserID: userID},
 	}
 }