diff --git a/CHANGELOG.md b/CHANGELOG.md
index dbba7b57a05..3887dac7287 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -58,7 +58,7 @@ To learn more about active deprecations, we recommend checking [GitHub Discussio
 
 ### New
 
-- TODO ([#XXX](https://github.com/kedacore/keda/issues/XXX))
+- **General**: Fix and optimize all webhook checkReplicas code ([#6344](https://github.com/kedacore/keda/pull/6344))
 
 #### Experimental
 
diff --git a/apis/keda/v1alpha1/scaledobject_types.go b/apis/keda/v1alpha1/scaledobject_types.go
index 37c5e640963..e40da6c51b2 100644
--- a/apis/keda/v1alpha1/scaledobject_types.go
+++ b/apis/keda/v1alpha1/scaledobject_types.go
@@ -239,13 +239,19 @@ func (so *ScaledObject) IsUsingModifiers() bool {
 
 // getHPAMinReplicas returns MinReplicas based on definition in ScaledObject or default value if not defined
 func (so *ScaledObject) GetHPAMinReplicas() *int32 {
-	if so.Spec.MinReplicaCount != nil && *so.Spec.MinReplicaCount > 0 {
+	if so.Spec.MinReplicaCount != nil {
 		return so.Spec.MinReplicaCount
 	}
 	tmp := defaultHPAMinReplicas
 	return &tmp
 }
 
+// GetDefaultHPAMinReplicas returns defaultHPAMinReplicas
+func (so *ScaledObject) GetDefaultHPAMinReplicas() *int32 {
+	tmp := defaultHPAMinReplicas
+	return &tmp
+}
+
 // getHPAMaxReplicas returns MaxReplicas based on definition in ScaledObject or default value if not defined
 func (so *ScaledObject) GetHPAMaxReplicas() int32 {
 	if so.Spec.MaxReplicaCount != nil {
@@ -254,21 +260,49 @@ func (so *ScaledObject) GetHPAMaxReplicas() int32 {
 	return defaultHPAMaxReplicas
 }
 
+// GetDefaultHPAMaxReplicas returns defaultHPAMaxReplicas
+func (so *ScaledObject) GetDefaultHPAMaxReplicas() int32 {
+	return defaultHPAMaxReplicas
+}
+
+// CheckReplicasNotNegative checks that Min/Max ReplicaCount defined in ScaledObject are not negative
+func (so *ScaledObject) CheckReplicasNotNegative(replicas ...int32) bool {
+	for _, replica := range replicas {
+		if replica < 0 {
+			return false
+		}
+	}
+	return true
+}
+
+// GetIdleReplicasIfDefined returns bool based on whether idleRelicas is defined
+func (so *ScaledObject) GetIdleReplicasIfDefined() bool {
+	return so.Spec.IdleReplicaCount != nil
+}
+
 // checkReplicaCountBoundsAreValid checks that Idle/Min/Max ReplicaCount defined in ScaledObject are correctly specified
 // i.e. that Min is not greater than Max or Idle greater or equal to Min
 func CheckReplicaCountBoundsAreValid(scaledObject *ScaledObject) error {
-	min := int32(0)
-	if scaledObject.Spec.MinReplicaCount != nil {
-		min = *scaledObject.GetHPAMinReplicas()
+	var idleReplicas *int32
+	minReplicas := *scaledObject.GetHPAMinReplicas()
+	maxReplicas := scaledObject.GetHPAMaxReplicas()
+	idleReplicasDefined := scaledObject.GetIdleReplicasIfDefined()
+	idleReplicas = scaledObject.Spec.IdleReplicaCount
+
+	if !scaledObject.CheckReplicasNotNegative(minReplicas, maxReplicas) {
+		return fmt.Errorf("MinReplicaCount=%d, MaxReplicaCount=%d must not be negative", minReplicas, maxReplicas)
+	}
+
+	if idleReplicasDefined && *idleReplicas < 0 {
+		return fmt.Errorf("IdleReplicaCount=%d must not be negative", *idleReplicas)
 	}
-	max := scaledObject.GetHPAMaxReplicas()
 
-	if min > max {
-		return fmt.Errorf("MinReplicaCount=%d must be less than MaxReplicaCount=%d", min, max)
+	if minReplicas > maxReplicas {
+		return fmt.Errorf("MinReplicaCount=%d must not be greater than MaxReplicaCount=%d", minReplicas, maxReplicas)
 	}
 
-	if scaledObject.Spec.IdleReplicaCount != nil && *scaledObject.Spec.IdleReplicaCount >= min {
-		return fmt.Errorf("IdleReplicaCount=%d must be less than MinReplicaCount=%d", *scaledObject.Spec.IdleReplicaCount, min)
+	if idleReplicasDefined && *idleReplicas >= minReplicas {
+		return fmt.Errorf("IdleReplicaCount=%d must be less than MinReplicaCount=%d", *scaledObject.Spec.IdleReplicaCount, minReplicas)
 	}
 
 	return nil
diff --git a/apis/keda/v1alpha1/scaledobject_webhook.go b/apis/keda/v1alpha1/scaledobject_webhook.go
index 41bd0355596..e4583730b15 100644
--- a/apis/keda/v1alpha1/scaledobject_webhook.go
+++ b/apis/keda/v1alpha1/scaledobject_webhook.go
@@ -184,7 +184,7 @@ func verifyReplicaCount(incomingSo *ScaledObject, action string, _ bool) error {
 		scaledobjectlog.WithValues("name", incomingSo.Name).Error(err, "validation error")
 		metricscollector.RecordScaledObjectValidatingErrors(incomingSo.Namespace, action, "incorrect-replicas")
 	}
-	return nil
+	return err
 }
 
 func verifyFallback(incomingSo *ScaledObject, action string, _ bool) error {
diff --git a/controllers/keda/hpa.go b/controllers/keda/hpa.go
index 5fb2c835eb7..88f794955f2 100644
--- a/controllers/keda/hpa.go
+++ b/controllers/keda/hpa.go
@@ -102,6 +102,19 @@ func (r *ScaledObjectReconciler) newHPAForScaledObject(ctx context.Context, logg
 	minReplicas := scaledObject.GetHPAMinReplicas()
 	maxReplicas := scaledObject.GetHPAMaxReplicas()
 
+	if *minReplicas == 0 {
+		minReplicas = scaledObject.GetDefaultHPAMinReplicas()
+	}
+
+	if maxReplicas == 0 {
+		maxReplicas = scaledObject.GetDefaultHPAMaxReplicas()
+	}
+
+	if *minReplicas < 0 || maxReplicas < 0 {
+		err := fmt.Errorf("MinReplicaCount=%d and MaxReplicaCount=%d must not be negative", minReplicas, maxReplicas)
+		return nil, err
+	}
+
 	pausedCount, err := executor.GetPausedReplicaCount(scaledObject)
 	if err != nil {
 		return nil, err