diff --git a/packages/backend/src/core/UtilityService.ts b/packages/backend/src/core/UtilityService.ts
index 09545b152f..24ded0677d 100644
--- a/packages/backend/src/core/UtilityService.ts
+++ b/packages/backend/src/core/UtilityService.ts
@@ -123,10 +123,22 @@ export class UtilityService {
 		return host;
 	}
 
+	private specialSuffix(hostname: string): string | null {
+		// masto.host provides domain names for its clients, we have to
+		// treat it as if it were a public suffix
+		const mastoHost = hostname.match(/\.?([a-zA-Z0-9-]+\.masto\.host)$/i);
+		if (mastoHost) {
+			return mastoHost[1];
+		}
+
+		return null;
+	}
+
 	@bindThis
 	public punyHostPSLDomain(url: string): string {
 		const urlObj = new URL(url);
-		const domain = psl.get(urlObj.hostname) ?? urlObj.hostname;
+		const hostname = urlObj.hostname;
+		const domain = this.specialSuffix(hostname) ?? psl.get(hostname) ?? hostname;
 		const host = `${this.toPuny(domain)}${urlObj.port.length > 0 ? ':' + urlObj.port : ''}`;
 		return host;
 	}
diff --git a/packages/backend/test/unit/UtilityService.ts b/packages/backend/test/unit/UtilityService.ts
index 837b55206e..d86e794f2f 100644
--- a/packages/backend/test/unit/UtilityService.ts
+++ b/packages/backend/test/unit/UtilityService.ts
@@ -35,5 +35,9 @@ describe('UtilityService', () => {
 			assert.equal(utilityService.punyHostPSLDomain('http://foo.github.io'), 'foo.github.io');
 			assert.equal(utilityService.punyHostPSLDomain('http://foo.bar.github.io'), 'bar.github.io');
 		});
+		test('special', () => {
+			assert.equal(utilityService.punyHostPSLDomain('http://foo.masto.host'), 'foo.masto.host');
+			assert.equal(utilityService.punyHostPSLDomain('http://foo.bar.masto.host'), 'bar.masto.host');
+		});
 	});
 });