Thompson's Lake Beach is the guarded swimming beach at Thompson's Lake Campground, the lakeside unit of John Boyd Thacher State Park run by the New York State Office of Parks, Recreation and Historic Preservation. It sits on the north end of Thompson's Lake in the Helderberg hilltowns of Albany County, about four miles back from the Helderberg Escarpment and roughly 18 miles southwest of Albany. The park address is 68 Thompsons Lake Road (NY 157), East Berne. Most of the shoreline around the rest of the lake is private cottages, so this is the one public place to swim on it.

From Albany, take NY 85 west to NY 157 and follow 157 past the Thacher overlooks to the Thompsons Lake Road entrance. Day visitors pay at the booth and park in the lot near the beach and playground. The park limits day-use parking to about 30 first-come spaces and bans parking along the campground roads, so on hot weekends the lot can fill by late morning. You're allowed to walk or bike in if it does. The beach is a short walk from the lot, with restrooms and changing tables close by.

The beach is a small strip of sand facing a roped swimming area on the north shore. Lifeguards are on duty during posted hours, and swimming is only allowed when they are. The season runs weekends only from late May to late June, then daily through Labor Day, 10 a.m. to 6 p.m. The water is calm lake water with no current, but at this elevation it stays cold well into June, and the shallows outside the swim area grow lily pads and weeds by late summer, so stay inside the ropes.

Entry is $7 per car, the standard state park vehicle fee. There is no separate charge for the beach. An Empire Pass covers the vehicle fee. Kayaks, tandem kayaks, canoes and stand-up paddleboards rent for $20 for the first two hours and $10 for each extra hour, but only while the beach is open and guards are present. Hours change year to year, and the park posted weekday closures of the swim area in the last week of August before reopening for Labor Day weekend, so call the campground at (518) 872-1674 before driving up late in the season.

A few rules catch people out. Pets are not allowed on guarded beaches in New York state parks (service animals excepted). Fishing is banned at the Emma Treadwell Thacher Nature Center shoreline. Anglers use the DEC fishing access site and boat launch at the south end of the lake instead. Swimming can be cancelled for storms or water quality results, and the beach does not open at all outside the posted season.

The campground has 140 wooded sites, a playground, volleyball, horseshoes and playing fields. Next to it, the Emma Treadwell Thacher Nature Center, opened in 2001, has exhibits and year-round programs, and trails run from the center down to the lake and through the surrounding woods and meadows. The cliffs, caves and escarpment views of the main Thacher Park area are a few minutes' drive east on NY 157.