diff --git a/internal/household/settings.go b/internal/household/settings.go
index 1e1df89..8e6342c 100644
--- a/internal/household/settings.go
+++ b/internal/household/settings.go
@@ -68,6 +68,11 @@ type SettingsPatch struct {
 	SuggestionsTime  *string
 	ChatAutosendDow  *int
 	ChatAutosendTime *string
+	// OnboardingCompleted, when true, records that the household finished
+	// onboarding: onboarding_completed_at becomes the time of the update,
+	// unless it is already set, when the first time is kept. Onboarding is
+	// never undone, so false is invalid.
+	OnboardingCompleted *bool
 }
 
 // Problem is one invalid field of a SettingsPatch, named by its JSON key.
@@ -208,6 +213,9 @@ func (s *SettingsService) Update(ctx context.Context, patch SettingsPatch, updat
 		setString(&p.SuggestionsTime, patch.SuggestionsTime)
 		setDow(&p.ChatAutosendDow, patch.ChatAutosendDow)
 		setString(&p.ChatAutosendTime, patch.ChatAutosendTime)
+		if patch.OnboardingCompleted != nil && !p.OnboardingCompletedAt.Valid {
+			p.OnboardingCompletedAt = sql.NullString{String: p.UpdatedAt, Valid: true}
+		}
 		row, err = q.UpsertHouseholdSettings(ctx, p)
 		if err != nil {
 			return fmt.Errorf("household: update settings: %w", err)
@@ -291,6 +299,9 @@ func (p SettingsPatch) validate() error {
 			fail(d.field, fmt.Sprintf("invalid day of the week %d (want 0-6, Sunday = 0)", *d.value))
 		}
 	}
+	if p.OnboardingCompleted != nil && !*p.OnboardingCompleted {
+		fail("onboarding_completed", "onboarding cannot be undone (want true)")
+	}
 	if len(problems) > 0 {
 		return &ValidationError{Problems: problems}
 	}
diff --git a/internal/household/settings_test.go b/internal/household/settings_test.go
index a01120c..9fca588 100644
--- a/internal/household/settings_test.go
+++ b/internal/household/settings_test.go
@@ -217,6 +217,39 @@ func TestUpdateAppliesPatchAndRecordsUpdatedBy(t *testing.T) {
 	}
 }
 
+func TestUpdateCompletesOnboardingOnce(t *testing.T) {
+	t.Parallel()
+	ctx := context.Background()
+	svc := seededService(t)
+
+	got, err := svc.Update(ctx, SettingsPatch{OnboardingCompleted: ptr(true)}, "sam")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got.OnboardingCompletedAt == nil || *got.OnboardingCompletedAt != got.UpdatedAt {
+		t.Fatalf("onboarding_completed_at = %v, want the update time %s", got.OnboardingCompletedAt, got.UpdatedAt)
+	}
+	first := *got.OnboardingCompletedAt
+
+	// Completing again keeps the first time.
+	got, err = svc.Update(ctx, SettingsPatch{OnboardingCompleted: ptr(true)}, "alex")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got.OnboardingCompletedAt == nil || *got.OnboardingCompletedAt != first {
+		t.Errorf("second completion: onboarding_completed_at = %v, want %s", got.OnboardingCompletedAt, first)
+	}
+
+	// Other changes leave it alone.
+	got, err = svc.Update(ctx, SettingsPatch{DinnerTime: ptr("18:00")}, "sam")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got.OnboardingCompletedAt == nil || *got.OnboardingCompletedAt != first {
+		t.Errorf("after another change: onboarding_completed_at = %v, want %s", got.OnboardingCompletedAt, first)
+	}
+}
+
 func TestUpdateRequiresMember(t *testing.T) {
 	t.Parallel()
 	ctx := context.Background()
@@ -259,6 +292,7 @@ func TestUpdateValidation(t *testing.T) {
 		{"dow 7", SettingsPatch{PlanDraftDow: ptr(7)}, []string{"plan_draft_dow"}},
 		{"dow -1", SettingsPatch{SuggestionsDow: ptr(-1)}, []string{"suggestions_dow"}},
 		{"dow 100", SettingsPatch{ChatAutosendDow: ptr(100)}, []string{"chat_autosend_dow"}},
+		{"onboarding undone", SettingsPatch{OnboardingCompleted: ptr(false)}, []string{"onboarding_completed"}},
 		{
 			"every problem is reported",
 			SettingsPatch{Timezone: ptr("Nowhere"), DinnerTime: ptr("5pm"), PlanDraftDow: ptr(9)},
diff --git a/internal/server/settings.go b/internal/server/settings.go
index a5f06b7..cce8413 100644
--- a/internal/server/settings.go
+++ b/internal/server/settings.go
@@ -37,6 +37,9 @@ type settingsPatch struct {
 	SuggestionsTime  *string `json:"suggestions_time"`
 	ChatAutosendDow  *int    `json:"chat_autosend_dow"`
 	ChatAutosendTime *string `json:"chat_autosend_time"`
+	// OnboardingCompleted true sets onboarding_completed_at, which is
+	// itself read-only.
+	OnboardingCompleted *bool `json:"onboarding_completed"`
 }
 
 func getHouseholdSettings(logger *slog.Logger, svc *household.SettingsService) http.HandlerFunc {
diff --git a/internal/server/settings_test.go b/internal/server/settings_test.go
index 8491eb0..08b7736 100644
--- a/internal/server/settings_test.go
+++ b/internal/server/settings_test.go
@@ -115,6 +115,19 @@ func TestPatchHouseholdSettings(t *testing.T) {
 	}
 }
 
+func TestPatchHouseholdSettingsCompletesOnboarding(t *testing.T) {
+	t.Parallel()
+
+	rec, body := serve(t, newTestRouter(t, "dev"), signedIn(http.MethodPatch, settingsPath,
+		strings.NewReader(`{"onboarding_completed":true}`)))
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200: %s", rec.Code, rec.Body)
+	}
+	if at, ok := body["onboarding_completed_at"].(string); !ok || at != body["updated_at"] {
+		t.Errorf("onboarding_completed_at = %#v, want the update time %#v", body["onboarding_completed_at"], body["updated_at"])
+	}
+}
+
 func TestPatchHouseholdSettingsRejected(t *testing.T) {
 	t.Parallel()
 
@@ -136,6 +149,8 @@ func TestPatchHouseholdSettingsRejected(t *testing.T) {
 		{"null value", `{"timezone":null}`, "invalid_request", "timezone"},
 		{"unknown field", `{"dinner":"17:30"}`, "invalid_request", "dinner"},
 		{"read-only field", `{"updated_by":"someone"}`, "invalid_request", "updated_by"},
+		{"read-only onboarding time", `{"onboarding_completed_at":"2026-10-06T12:00:00.000Z"}`, "invalid_request", "onboarding_completed_at"},
+		{"onboarding undone", `{"onboarding_completed":false}`, "invalid_settings", "onboarding_completed"},
 		{"not an object", `["timezone"]`, "invalid_request", "JSON object"},
 		{"null body", `null`, "invalid_request", "JSON object"},
 		{"trailing data", `{"dinner_time":"18:00"} {}`, "invalid_request", "JSON object"},