src/cmd/internal/test2json/test2json.go             |  7 ++++++-
 src/cmd/internal/test2json/testdata/issue23920.json | 14 ++++++++++++++
 src/cmd/internal/test2json/testdata/issue23920.test |  7 +++++++

diff --git a/src/cmd/internal/test2json/test2json.go b/src/cmd/internal/test2json/test2json.go
index 483fb1de523cf0eee53c096b3beb65fe31b0faf7..f8052136be625025bd47f8e2d771e837c44794e6 100644
--- a/src/cmd/internal/test2json/test2json.go
+++ b/src/cmd/internal/test2json/test2json.go
@@ -175,6 +175,7 @@
 	// "=== RUN   "
 	// "=== PAUSE "
 	// "=== CONT  "
+	actionColon := false
 	origLine := line
 	ok := false
 	indent := 0
@@ -196,6 +197,7 @@ 			indent++
 		}
 		for _, magic := range reports {
 			if bytes.HasPrefix(line, magic) {
+				actionColon = true
 				ok = true
 				break
 			}
@@ -209,7 +211,10 @@ 		return
 	}
 
 	// Parse out action and test name.
-	i := bytes.IndexByte(line, ':') + 1
+	i := 0
+	if actionColon {
+		i = bytes.IndexByte(line, ':') + 1
+	}
 	if i == 0 {
 		i = len(updates[0])
 	}
diff --git a/src/cmd/internal/test2json/testdata/issue23920.json b/src/cmd/internal/test2json/testdata/issue23920.json
new file mode 100644
index 0000000000000000000000000000000000000000..28f7bd56ac39fb62bb24e967a8d3b1f27d00e9d8
--- /dev/null
+++ b/src/cmd/internal/test2json/testdata/issue23920.json
@@ -0,0 +1,14 @@
+{"Action":"run","Test":"TestWithColons"}
+{"Action":"output","Test":"TestWithColons","Output":"=== RUN   TestWithColons\n"}
+{"Action":"run","Test":"TestWithColons/[::1]"}
+{"Action":"output","Test":"TestWithColons/[::1]","Output":"=== RUN   TestWithColons/[::1]\n"}
+{"Action":"run","Test":"TestWithColons/127.0.0.1:0"}
+{"Action":"output","Test":"TestWithColons/127.0.0.1:0","Output":"=== RUN   TestWithColons/127.0.0.1:0\n"}
+{"Action":"output","Test":"TestWithColons","Output":"--- PASS: TestWithColons (0.00s)\n"}
+{"Action":"output","Test":"TestWithColons/[::1]","Output":"    --- PASS: TestWithColons/[::1] (0.00s)\n"}
+{"Action":"pass","Test":"TestWithColons/[::1]"}
+{"Action":"output","Test":"TestWithColons/127.0.0.1:0","Output":"    --- PASS: TestWithColons/127.0.0.1:0 (0.00s)\n"}
+{"Action":"pass","Test":"TestWithColons/127.0.0.1:0"}
+{"Action":"pass","Test":"TestWithColons"}
+{"Action":"output","Output":"PASS\n"}
+{"Action":"pass"}
diff --git a/src/cmd/internal/test2json/testdata/issue23920.test b/src/cmd/internal/test2json/testdata/issue23920.test
new file mode 100644
index 0000000000000000000000000000000000000000..43bf0580343f489e2d650947cb2cb31b7397d9a6
--- /dev/null
+++ b/src/cmd/internal/test2json/testdata/issue23920.test
@@ -0,0 +1,7 @@
+=== RUN   TestWithColons
+=== RUN   TestWithColons/[::1]
+=== RUN   TestWithColons/127.0.0.1:0
+--- PASS: TestWithColons (0.00s)
+    --- PASS: TestWithColons/[::1] (0.00s)
+    --- PASS: TestWithColons/127.0.0.1:0 (0.00s)
+PASS