diff --git a/api/types/helmchartargs.go b/api/types/helmchartargs.go
index 86afc52cf2..2ec7d8efde 100644
--- a/api/types/helmchartargs.go
+++ b/api/types/helmchartargs.go
@@ -148,6 +148,11 @@ func makeHelmChartFromHca(old *HelmChartArgs) (c HelmChart) {
 	c.ValuesInline = old.ValuesLocal
 	c.ValuesMerge = old.ValuesMerge
 	c.ReleaseName = old.ReleaseName
+	// ReleaseNamespace maps onto Namespace in the (non-deprecated)
+	// HelmChart type. Forgetting this mapping is why releaseNamespace
+	// set under the deprecated helmChartInflationGenerator field is
+	// silently ignored — see kubernetes-sigs/kustomize#4593.
+	c.Namespace = old.ReleaseNamespace
 	return
 }
 
diff --git a/api/types/helmchartargs_test.go b/api/types/helmchartargs_test.go
index fd9a7a79a1..1c011d92e8 100644
--- a/api/types/helmchartargs_test.go
+++ b/api/types/helmchartargs_test.go
@@ -102,3 +102,23 @@ func TestAsHelmArgs(t *testing.T) {
 				"--devel"})
 	})
 }
+
+// Regression test for https://github.com/kubernetes-sigs/kustomize/issues/4593.
+// HelmChartArgs.ReleaseNamespace was not copied into HelmChart.Namespace by
+// makeHelmChartFromHca, so kustomizations that used the deprecated
+// helmChartInflationGenerator with releaseNamespace silently rendered resources
+// into the "default" namespace.
+func TestSplitHelmParametersPropagatesReleaseNamespace(t *testing.T) {
+	args := []types.HelmChartArgs{{
+		ChartName:        "nats",
+		ChartVersion:     "v0.13.1",
+		ReleaseName:      "nats",
+		ReleaseNamespace: "custom-ns",
+	}}
+
+	charts, _ := types.SplitHelmParameters(args)
+
+	require.Len(t, charts, 1)
+	require.Equal(t, "custom-ns", charts[0].Namespace,
+		"ReleaseNamespace from the deprecated HelmChartArgs must map onto HelmChart.Namespace")
+}