diff --git a/app/features/tournament-bracket/actions/to.$id.brackets.server.test.ts b/app/features/tournament-bracket/actions/to.$id.brackets.server.test.ts
new file mode 100644
index 000000000..68c6901ca
--- /dev/null
+++ b/app/features/tournament-bracket/actions/to.$id.brackets.server.test.ts
@@ -0,0 +1,144 @@
+import { beforeEach, describe, expect, test, vi } from "vitest";
+
+vi.mock("~/features/chat/ChatSystemMessage.server", () => ({
+	send: vi.fn(),
+	notifyNotificationsChanged: vi.fn(),
+	notifyRoomsChangedByRoomIds: vi.fn(),
+}));
+
+import * as TournamentFactory from "~/db/seed/factories/TournamentFactory";
+import * as TournamentTeamFactory from "~/db/seed/factories/TournamentTeamFactory";
+import * as UserFactory from "~/db/seed/factories/UserFactory";
+import type { TournamentSettings } from "~/db/tables-json";
+import { tournamentFromDB } from "~/features/tournament-bracket/core/Tournament.server";
+import type { bracketSchema } from "~/features/tournament-bracket/tournament-bracket-schemas";
+import invariant from "~/utils/invariant";
+import { wrappedAction } from "~/utils/Test";
+import { action } from "./to.$id.brackets.server";
+
+const bracketsAction = wrappedAction<typeof bracketSchema>({
+	action,
+	isJsonSubmission: true,
+});
+
+const TEAM_COUNT = 8;
+
+/** Two pools of four, the top two of each pool advancing to a single group swiss. */
+const POOLS_TO_SWISS: TournamentSettings["bracketProgression"] = [
+	{
+		name: "Pools",
+		type: "round_robin",
+		requiresCheckIn: false,
+		settings: { teamsPerGroup: 4 },
+	},
+	{
+		name: "Swiss",
+		type: "swiss",
+		requiresCheckIn: false,
+		settings: { groupCount: 1, roundCount: 3 },
+		sources: [{ bracketIdx: 0, placements: [1, 2] }],
+	},
+];
+
+const users = UserFactory.pool();
+const organizerId = () => users.id(1);
+
+describe("Brackets action UNADVANCE_BRACKET", () => {
+	beforeEach(async () => {
+		await users.create(TEAM_COUNT);
+	});
+
+	test("deletes a swiss round of a follow-up swiss bracket whose own follow-ups have not started", async () => {
+		const tournament = await TournamentFactory.createPlayed(
+			{
+				authorId: organizerId(),
+				bracketProgression: POOLS_TO_SWISS,
+				minMembersPerTeam: 1,
+			},
+			{
+				teamRosters: users.ids(TEAM_COUNT).map((userId) => [userId]),
+				playedOut: 0,
+			},
+		);
+		await TournamentFactory.startBracket(tournament.id, { bracketIdx: 1 });
+
+		const before = await tournamentFromDB(tournament.id);
+		const swiss = before.bracketByIdx(1);
+		invariant(swiss && !swiss.preview);
+		const group = swiss.data.group[0];
+		const firstRound = swiss.data.round.find(
+			(round) => round.groupId === group.id && round.number === 1,
+		);
+		invariant(firstRound);
+		expect(
+			swiss.data.match.filter((match) => match.roundId === firstRound.id)
+				.length,
+		).toBeGreaterThan(0);
+
+		const response = await bracketsAction(
+			{
+				_action: "UNADVANCE_BRACKET",
+				bracketIdx: 1,
+				groupId: group.id,
+				roundId: firstRound.id,
+			},
+			{ user: organizerId(), params: { id: String(tournament.id) } },
+		);
+
+		expect(
+			response instanceof Response ? response.headers.get("Location") : null,
+		).toBeNull();
+
+		const after = await tournamentFromDB(tournament.id);
+		const swissAfter = after.bracketByIdx(1);
+		invariant(swissAfter);
+		expect(
+			swissAfter.data.match.filter((match) => match.roundId === firstRound.id),
+		).toHaveLength(0);
+	});
+
+	test("deletes a swiss round of a starting swiss bracket", async () => {
+		const tournament = await TournamentFactory.create({
+			authorId: organizerId(),
+			bracketProgression: [
+				{
+					name: "Swiss",
+					type: "swiss",
+					requiresCheckIn: false,
+					settings: { groupCount: 1, roundCount: 3 },
+				},
+			],
+			minMembersPerTeam: 1,
+		});
+		for (const userId of users.ids(TEAM_COUNT)) {
+			await TournamentTeamFactory.create(
+				{ tournamentId: tournament.id, memberUserIds: [userId] },
+				{ isCheckedIn: true },
+			);
+		}
+		await TournamentFactory.startBracket(tournament.id, { bracketIdx: 0 });
+
+		const before = await tournamentFromDB(tournament.id);
+		const swiss = before.bracketByIdx(0);
+		invariant(swiss && !swiss.preview);
+		const group = swiss.data.group[0];
+		const firstRound = swiss.data.round.find(
+			(round) => round.groupId === group.id && round.number === 1,
+		);
+		invariant(firstRound);
+
+		const response = await bracketsAction(
+			{
+				_action: "UNADVANCE_BRACKET",
+				bracketIdx: 0,
+				groupId: group.id,
+				roundId: firstRound.id,
+			},
+			{ user: organizerId(), params: { id: String(tournament.id) } },
+		);
+
+		expect(
+			response instanceof Response ? response.headers.get("Location") : null,
+		).toBeNull();
+	});
+});
diff --git a/app/features/tournament-bracket/actions/to.$id.brackets.server.ts b/app/features/tournament-bracket/actions/to.$id.brackets.server.ts
index 5f2e7fa02..6c416f7d3 100644
--- a/app/features/tournament-bracket/actions/to.$id.brackets.server.ts
+++ b/app/features/tournament-bracket/actions/to.$id.brackets.server.ts
@@ -262,7 +262,7 @@ export const action: ActionFunction = async ({ params, request }) => {
 				bracket.type === "swiss",
 				"Can't unadvance non-swiss bracket",
 			);
-			errorToastIfFalsyNoFollowUpBrackets(tournament);
+			errorToastIfFalsyNoFollowUpBrackets(tournament, data.bracketIdx);
 
 			await BracketRepository.deleteRoundMatches({
 				groupId: data.groupId,
@@ -340,9 +340,12 @@ export const action: ActionFunction = async ({ params, request }) => {
 	return null;
 };
 
-function errorToastIfFalsyNoFollowUpBrackets(tournament: Tournament) {
+function errorToastIfFalsyNoFollowUpBrackets(
+	tournament: Tournament,
+	bracketIdx: number,
+) {
 	const followUpBrackets = tournament.brackets.filter((b) =>
-		b.sources?.some((source) => source.bracketIdx === 0),
+		b.sources?.some((source) => source.bracketIdx === bracketIdx),
 	);
 
 	errorToastIfFalsy(
diff --git a/changelog/2026-09-02-swiss-round-delete.md b/changelog/2026-09-02-swiss-round-delete.md
new file mode 100644
index 000000000..3c6719874
--- /dev/null
+++ b/changelog/2026-09-02-swiss-round-delete.md
@@ -0,0 +1,5 @@
+---
+navItem: calendar
+type: bug
+---
+Deleting a round of a Swiss bracket is no longer blocked when the Swiss bracket is not the tournament's first bracket