diff --git a/pkg/apis/serving/register.go b/pkg/apis/serving/register.go
index da8f024ecb42..9373f9553a98 100644
--- a/pkg/apis/serving/register.go
+++ b/pkg/apis/serving/register.go
@@ -56,4 +56,8 @@ const (
 	// ConfigurationGenerationLabelKey is the label key attached to a Revision indicating the
 	// generation of the Configuration that created this revision
 	ConfigurationGenerationLabelKey = GroupName + "/configurationGeneration"
+
+	// BuildHashLabelKey is the label key attached to a Build indicating the
+	// hash of the spec from which they were created.
+	BuildHashLabelKey = GroupName + "/buildHash"
 )
diff --git a/pkg/reconciler/v1alpha1/configuration/resources/build.go b/pkg/reconciler/v1alpha1/configuration/resources/build.go
index 6f8e101e2942..5e566edeea61 100644
--- a/pkg/reconciler/v1alpha1/configuration/resources/build.go
+++ b/pkg/reconciler/v1alpha1/configuration/resources/build.go
@@ -17,6 +17,8 @@ limitations under the License.
 package resources
 
 import (
+	"crypto/sha256"
+	"encoding/hex"
 	"encoding/json"
 
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
@@ -24,6 +26,7 @@ import (
 
 	buildv1alpha1 "github.com/knative/build/pkg/apis/build/v1alpha1"
 	"github.com/knative/pkg/kmeta"
+	"github.com/knative/serving/pkg/apis/serving"
 	"github.com/knative/serving/pkg/apis/serving/v1alpha1"
 	"github.com/knative/serving/pkg/reconciler/v1alpha1/configuration/resources/names"
 )
@@ -48,6 +51,19 @@ func MakeBuild(config *v1alpha1.Configuration) *unstructured.Unstructured {
 
 		u = MustToUnstructured(b)
 	}
+	// After calling `As()` we can be sure that `.Raw` is populated.
+
+	// Compute the hash of the current build's spec.
+	sum := sha256.Sum256(config.Spec.Build.Raw)
+	h := hex.EncodeToString(sum[:])
+
+	// Put it into a label for later lookups.
+	l := u.GetLabels()
+	if l == nil {
+		l = make(map[string]string)
+	}
+	l[serving.BuildHashLabelKey] = h[:63] // Labels can only be 63 characters.
+	u.SetLabels(l)
 
 	u.SetNamespace(config.Namespace)
 	u.SetName(names.Build(config))
diff --git a/pkg/reconciler/v1alpha1/configuration/resources/build_test.go b/pkg/reconciler/v1alpha1/configuration/resources/build_test.go
index 3bd42dfb0714..1b51815c48c3 100644
--- a/pkg/reconciler/v1alpha1/configuration/resources/build_test.go
+++ b/pkg/reconciler/v1alpha1/configuration/resources/build_test.go
@@ -89,6 +89,9 @@ func TestBuilds(t *testing.T) {
 					"controller":         true,
 					"blockOwnerDeletion": true,
 				}},
+				"labels": map[string]interface{}{
+					"serving.knative.dev/buildHash": "2ee4528bee48a78637ec374eb58cb1977b9611b85545f8b91884ff80b8d9472",
+				},
 				"creationTimestamp": nil,
 			},
 			"spec": map[string]interface{}{
@@ -146,6 +149,9 @@ func TestBuilds(t *testing.T) {
 					"controller":         true,
 					"blockOwnerDeletion": true,
 				}},
+				"labels": map[string]interface{}{
+					"serving.knative.dev/buildHash": "934e535117334c700c3a132e5e9dfc4276974cf2c9d6fe8f09c961f8e058933",
+				},
 				"creationTimestamp": nil,
 			},
 			"spec": map[string]interface{}{