diff --git a/doc/go1.12.html b/doc/go1.12.html
index 5cd35b9..0f076e3 100644
--- a/doc/go1.12.html
+++ b/doc/go1.12.html
@@ -388,15 +388,25 @@
 <h3 id="tls_1_3">TLS 1.3</h3>
 
 <p>
-  Go 1.12 adds support in the <code>crypto/tls</code> package for TLS 1.3 as
-  specified in <a href="https://www.rfc-editor.org/info/rfc8446">RFC 8446</a>.
+  Go 1.12 adds opt-in support for TLS 1.3 in the <code>crypto/tls</code> package as
+  specified by <a href="https://www.rfc-editor.org/info/rfc8446">RFC 8446</a>. It can
+  be enabled by adding the value <code>tls13=1</code> to the <code>GODEBUG</code>
+  environment variable. It will be enabled by default in Go 1.13.
+</p>
 
-  Programs that did not set an explicit <code>MaxVersion</code> in
-  <a href="/pkg/crypto/tls/#Config"><code>Config</code></a> will automatically negotiate
-  TLS 1.3 if available. All TLS 1.2 features except <code>TLSUnique</code> in
+<p>
+  To negotiate TLS 1.3, make sure you do not set an explicit <code>MaxVersion</code> in
+  <a href="/pkg/crypto/tls/#Config"><code>Config</code></a> and run your program with
+  the environment variable <code>GODEBUG=tls13=1</code> set.
+</p>
+
+<p>
+  All TLS 1.2 features except <code>TLSUnique</code> in
   <a href="/pkg/crypto/tls/#ConnectionState"><code>ConnectionState</code></a>
   and renegotiation are available in TLS 1.3 and provide equivalent or
-  better security and performance.
+  better security and performance. Note that even though TLS 1.3 is backwards
+  compatible with previous versions, certain legacy systems might not work
+  correctly when attempting to negotiate it.
 </p>
 
 <p>
diff --git a/src/crypto/tls/common.go b/src/crypto/tls/common.go
index 59d5507..0b08700 100644
--- a/src/crypto/tls/common.go
+++ b/src/crypto/tls/common.go
@@ -16,6 +16,7 @@
 	"io"
 	"math/big"
 	"net"
+	"os"
 	"strings"
 	"sync"
 	"time"
@@ -775,11 +776,53 @@
 		if isClient && v < VersionTLS10 {
 			continue
 		}
+		// TLS 1.3 is opt-in in Go 1.12.
+		if v == VersionTLS13 && !isTLS13Supported() {
+			continue
+		}
 		versions = append(versions, v)
 	}
 	return versions
 }
 
+// tls13Support caches the result for isTLS13Supported.
+var tls13Support struct {
+	sync.Once
+	cached bool
+}
+
+// isTLS13Supported returns whether the program opted into TLS 1.3 via
+// GODEBUG=tls13=1. It's cached after the first execution.
+func isTLS13Supported() bool {
+	tls13Support.Do(func() {
+		tls13Support.cached = goDebugString("tls13") == "1"
+	})
+	return tls13Support.cached
+}
+
+// goDebugString returns the value of the named GODEBUG key.
+// GODEBUG is of the form "key=val,key2=val2".
+func goDebugString(key string) string {
+	s := os.Getenv("GODEBUG")
+	for i := 0; i < len(s)-len(key)-1; i++ {
+		if i > 0 && s[i-1] != ',' {
+			continue
+		}
+		afterKey := s[i+len(key):]
+		if afterKey[0] != '=' || s[i:i+len(key)] != key {
+			continue
+		}
+		val := afterKey[1:]
+		for i, b := range val {
+			if b == ',' {
+				return val[:i]
+			}
+		}
+		return val
+	}
+	return ""
+}
+
 func (c *Config) maxSupportedVersion(isClient bool) uint16 {
 	supportedVersions := c.supportedVersions(isClient)
 	if len(supportedVersions) == 0 {
diff --git a/src/crypto/tls/tls_test.go b/src/crypto/tls/tls_test.go
index 00bb6e4..9c26769 100644
--- a/src/crypto/tls/tls_test.go
+++ b/src/crypto/tls/tls_test.go
@@ -18,10 +18,18 @@
 	"os"
 	"reflect"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 )
 
+func init() {
+	// TLS 1.3 is opt-in for Go 1.12, but we want to run most tests with it enabled.
+	// TestTLS13Switch below tests the disabled behavior. See Issue 30055.
+	tls13Support.Do(func() {}) // defuse the sync.Once
+	tls13Support.cached = true
+}
+
 var rsaCertPEM = `-----BEGIN CERTIFICATE-----
 MIIB0zCCAX2gAwIBAgIJAI/M7BYjwB+uMA0GCSqGSIb3DQEBBQUAMEUxCzAJBgNV
 BAYTAkFVMRMwEQYDVQQIDApTb21lLVN0YXRlMSEwHwYDVQQKDBhJbnRlcm5ldCBX
@@ -1076,18 +1084,47 @@
 		VersionSSL30,
 	}
 
-	ss, cs, err := testHandshake(t, testConfig, testConfig)
+	expectVersion(t, testConfig, testConfig, VersionTLS12)
+}
+
+func expectVersion(t *testing.T, clientConfig, serverConfig *Config, v uint16) {
+	ss, cs, err := testHandshake(t, clientConfig, serverConfig)
 	if err != nil {
-		t.Fatalf("Handshake failed when support for TLS 1.3 was dropped: %v", err)
+		t.Fatalf("Handshake failed: %v", err)
 	}
-	if ss.Version != VersionTLS12 {
-		t.Errorf("Server negotiated version %x, expected %x", cs.Version, VersionTLS12)
+	if ss.Version != v {
+		t.Errorf("Server negotiated version %x, expected %x", cs.Version, v)
 	}
-	if cs.Version != VersionTLS12 {
-		t.Errorf("Client negotiated version %x, expected %x", cs.Version, VersionTLS12)
+	if cs.Version != v {
+		t.Errorf("Client negotiated version %x, expected %x", cs.Version, v)
 	}
 }
 
+// TestTLS13Switch checks the behavior of GODEBUG=tls13=[0|1]. See Issue 30055.
+func TestTLS13Switch(t *testing.T) {
+	defer func(savedGODEBUG string) {
+		os.Setenv("GODEBUG", savedGODEBUG)
+	}(os.Getenv("GODEBUG"))
+
+	os.Setenv("GODEBUG", "tls13=0")
+	tls13Support.Once = sync.Once{} // reset the cache
+
+	tls12Config := testConfig.Clone()
+	tls12Config.MaxVersion = VersionTLS12
+	expectVersion(t, testConfig, testConfig, VersionTLS12)
+	expectVersion(t, tls12Config, testConfig, VersionTLS12)
+	expectVersion(t, testConfig, tls12Config, VersionTLS12)
+	expectVersion(t, tls12Config, tls12Config, VersionTLS12)
+
+	os.Setenv("GODEBUG", "tls13=1")
+	tls13Support.Once = sync.Once{} // reset the cache
+
+	expectVersion(t, testConfig, testConfig, VersionTLS13)
+	expectVersion(t, tls12Config, testConfig, VersionTLS12)
+	expectVersion(t, testConfig, tls12Config, VersionTLS12)
+	expectVersion(t, tls12Config, tls12Config, VersionTLS12)
+}
+
 // Issue 28744: Ensure that we don't modify memory
 // that Config doesn't own such as Certificates.
 func TestBuildNameToCertificate_doesntModifyCertificates(t *testing.T) {