diff --git a/pkg/apis/autoscaling/v1alpha1/pa_lifecycle.go b/pkg/apis/autoscaling/v1alpha1/pa_lifecycle.go
index 6606ac54058d..3ce6d98ad75e 100644
--- a/pkg/apis/autoscaling/v1alpha1/pa_lifecycle.go
+++ b/pkg/apis/autoscaling/v1alpha1/pa_lifecycle.go
@@ -191,6 +191,12 @@ func (pas *PodAutoscalerStatus) CanMarkInactive(idlePeriod time.Duration) bool {
 	return pas.inStatusFor(corev1.ConditionTrue, idlePeriod)
 }
 
+// IsActivatingTimeout checks whether the pod autoscaler has been in activating
+// for at least the specified timeout period.
+func (pas *PodAutoscalerStatus) IsActivatingTimeout(activatingTimeout time.Duration) bool {
+	return pas.inStatusFor(corev1.ConditionUnknown, activatingTimeout)
+}
+
 // inStatusFor returns true if the PodAutoscalerStatus's Active condition has stayed in
 // the specified status for at least the specified duration. Otherwise it returns false,
 // including when the status is undetermined (Active condition is not found.)
diff --git a/pkg/autoscaler/config.go b/pkg/autoscaler/config.go
index 88e68a2725f7..6e23c291b910 100644
--- a/pkg/autoscaler/config.go
+++ b/pkg/autoscaler/config.go
@@ -51,6 +51,7 @@ type Config struct {
 	TickInterval time.Duration
 
 	ScaleToZeroGracePeriod time.Duration
+	ActivatingTimeout      time.Duration
 }
 
 // TargetConcurrency calculates the target concurrency for a given container-concurrency
@@ -142,6 +143,10 @@ func NewConfigFromMap(data map[string]string) (*Config, error) {
 		key:          "tick-interval",
 		field:        &lc.TickInterval,
 		defaultValue: 2 * time.Second,
+	}, {
+		key:          "activating-timeout",
+		field:        &lc.ActivatingTimeout,
+		defaultValue: 2 * time.Minute,
 	}} {
 		if raw, ok := data[dur.key]; !ok {
 			*dur.field = dur.defaultValue
diff --git a/pkg/autoscaler/config_test.go b/pkg/autoscaler/config_test.go
index b6765df2892c..98df9b2300da 100644
--- a/pkg/autoscaler/config_test.go
+++ b/pkg/autoscaler/config_test.go
@@ -79,6 +79,7 @@ func TestNewConfig(t *testing.T) {
 			"tick-interval":                           "2s",
 			"panic-window-percentage":                 "10",
 			"panic-threshold-percentage":              "200",
+			"activating-timeout":                      "2m",
 		},
 		want: &Config{
 			EnableScaleToZero:                    true,
@@ -91,6 +92,7 @@ func TestNewConfig(t *testing.T) {
 			TickInterval:                         2 * time.Second,
 			PanicWindowPercentage:                10.0,
 			PanicThresholdPercentage:             200.0,
+			ActivatingTimeout:                    2 * time.Minute,
 		},
 	}, {
 		name: "with toggles on",
@@ -104,6 +106,7 @@ func TestNewConfig(t *testing.T) {
 			"tick-interval":                           "2s",
 			"panic-window-percentage":                 "10",
 			"panic-threshold-percentage":              "200",
+			"activating-timeout":                      "2m",
 		},
 		want: &Config{
 			EnableScaleToZero:                    true,
@@ -116,6 +119,7 @@ func TestNewConfig(t *testing.T) {
 			TickInterval:                         2 * time.Second,
 			PanicWindowPercentage:                10.0,
 			PanicThresholdPercentage:             200.0,
+			ActivatingTimeout:                    2 * time.Minute,
 		},
 	}, {
 		name: "with toggles on strange casing",
@@ -129,6 +133,7 @@ func TestNewConfig(t *testing.T) {
 			"tick-interval":                           "2s",
 			"panic-window-percentage":                 "10",
 			"panic-threshold-percentage":              "200",
+			"activating-timeout":                      "2m",
 		},
 		want: &Config{
 			EnableScaleToZero:                    true,
@@ -141,6 +146,7 @@ func TestNewConfig(t *testing.T) {
 			TickInterval:                         2 * time.Second,
 			PanicWindowPercentage:                10.0,
 			PanicThresholdPercentage:             200.0,
+			ActivatingTimeout:                    2 * time.Minute,
 		},
 	}, {
 		name: "with toggles explicitly off",
@@ -154,6 +160,7 @@ func TestNewConfig(t *testing.T) {
 			"tick-interval":                           "2s",
 			"panic-window-percentage":                 "10",
 			"panic-threshold-percentage":              "200",
+			"activating-timeout":                      "2m",
 		},
 		want: &Config{
 			ContainerConcurrencyTargetPercentage: 0.5,
@@ -165,6 +172,7 @@ func TestNewConfig(t *testing.T) {
 			TickInterval:                         2 * time.Second,
 			PanicWindowPercentage:                10.0,
 			PanicThresholdPercentage:             200.0,
+			ActivatingTimeout:                    2 * time.Minute,
 		},
 	}, {
 		name: "with explicit grace period",
@@ -179,6 +187,7 @@ func TestNewConfig(t *testing.T) {
 			"tick-interval":                           "2s",
 			"panic-window-percentage":                 "10",
 			"panic-threshold-percentage":              "200",
+			"activating-timeout":                      "2m",
 		},
 		want: &Config{
 			ContainerConcurrencyTargetPercentage: 0.5,
@@ -190,6 +199,7 @@ func TestNewConfig(t *testing.T) {
 			TickInterval:                         2 * time.Second,
 			PanicWindowPercentage:                10.0,
 			PanicThresholdPercentage:             200.0,
+			ActivatingTimeout:                    2 * time.Minute,
 		},
 	}, {
 		name: "malformed float",
@@ -202,6 +212,7 @@ func TestNewConfig(t *testing.T) {
 			"tick-interval":                           "2s",
 			"panic-window-percentage":                 "10",
 			"panic-threshold-percentage":              "200",
+			"activating-timeout":                      "2m",
 		},
 		wantErr: true,
 	}, {
@@ -215,6 +226,7 @@ func TestNewConfig(t *testing.T) {
 			"tick-interval":                           "2s",
 			"panic-window-percentage":                 "10",
 			"panic-threshold-percentage":              "200",
+			"activating-timeout":                      "2m",
 		},
 		wantErr: true,
 	}}
diff --git a/pkg/reconciler/autoscaling/kpa/kpa.go b/pkg/reconciler/autoscaling/kpa/kpa.go
index 2a04e5675670..47a97127682b 100644
--- a/pkg/reconciler/autoscaling/kpa/kpa.go
+++ b/pkg/reconciler/autoscaling/kpa/kpa.go
@@ -214,6 +214,12 @@ func (c *Reconciler) reconcile(ctx context.Context, pa *pav1alpha1.PodAutoscaler
 		return perrors.Wrap(err, "error reporting metrics")
 	}
 
+	if pa.Status.IsActivating() {
+		timeout := config.FromContext(ctx).Autoscaler.ActivatingTimeout
+		logger.Infof("enqueue delay for check activating timeout after %v.", timeout)
+		c.scaler.enqueueCB(pa, timeout)
+	}
+
 	// computeActiveCondition decides if we need to change the SKS mode,
 	// and returns true if the status has changed.
 	if changed := computeActiveCondition(pa, want, got); changed {
diff --git a/pkg/reconciler/autoscaling/kpa/scaler.go b/pkg/reconciler/autoscaling/kpa/scaler.go
index 6c35ba2e0f41..c00c3848ddf9 100644
--- a/pkg/reconciler/autoscaling/kpa/scaler.go
+++ b/pkg/reconciler/autoscaling/kpa/scaler.go
@@ -253,13 +253,19 @@ func (ks *scaler) applyScale(ctx context.Context, pa *pav1alpha1.PodAutoscaler,
 // Scale attempts to scale the given PA's target reference to the desired scale.
 func (ks *scaler) Scale(ctx context.Context, pa *pav1alpha1.PodAutoscaler, desiredScale int32) (int32, error) {
 	logger := logging.FromContext(ctx)
+	autoscalerConfig := config.FromContext(ctx).Autoscaler
 
 	if desiredScale < 0 {
-		logger.Debug("Metrics are not yet being collected.")
-		return desiredScale, nil
+		// check is activating timeout
+		if pa.Status.IsActivatingTimeout(autoscalerConfig.ActivatingTimeout) {
+			logger.Infof("Activating pa timeout after %v", autoscalerConfig.ActivatingTimeout)
+		} else {
+			logger.Debug("Metrics are not yet being collected.")
+			return desiredScale, nil
+		}
 	}
 
-	desiredScale, shouldApplyScale := ks.handleScaleToZero(pa, desiredScale, config.FromContext(ctx).Autoscaler)
+	desiredScale, shouldApplyScale := ks.handleScaleToZero(pa, desiredScale, autoscalerConfig)
 	if !shouldApplyScale {
 		return desiredScale, nil
 	}