Portage Lake Beach is the public swimming beach in the Portage Lake day-use area of Waterloo Recreation Area, a state recreation area run by the Michigan Department of Natural Resources. At more than 20,000 acres, Waterloo is the largest state park in the Lower Peninsula, spread across Jackson and Washtenaw counties between Jackson and Chelsea. The beach sits on the south shore of Big Portage Lake, the largest of the park's 11 inland lakes, about 9 kilometers (6 miles) north of the village of Grass Lake and closer still to the small community of Munith.

The day-use area is reached from Seymour Road, between Mt. Hope Road and Race Road, a few miles north of I-94. The Portage Lake Modern Campground (11500 Seymour Road, Grass Lake) shares the same entrance area, and campers walk to the beach on a paved path. Day visitors park in the lots above the beach next to the picnic grounds, playground and beach house. A Recreation Passport is required for every motor vehicle entering a Michigan state park. For Michigan-registered vehicles it is an annual pass only: $15 when added at license plate renewal through the Secretary of State, with a $5 convenience fee if bought separately at the park. Out-of-state vehicles pay $12 for a day pass (good for entry and re-entry at any state park that day) or $42 for an annual pass. People who walk or bike in do not need a passport. There is no separate charge for the beach itself.

The swim area is a designated beach on a sandy stretch of shoreline, with lawn and shade trees on the slope behind it. It is still lake water with no current to speak of, cool in early June and at its warmest from mid-July through August. The DNR lists no lifeguard service, so swimming is at your own risk and children need an adult watching them. Local listings give the day-use hours as 8 a.m. to 10 p.m. (one older park guide lists the beach itself as 8 a.m. to 8 p.m.), so check with park headquarters at 734-475-8307 if you plan an evening swim. The beach house has restrooms, changing stalls and a concession store.

This is the only beach in Waterloo open to day visitors. The park's other designated swim area, at Sugarloaf Lake, is reserved for registered campers in the Sugarloaf Campground. Busy summer weekends can fill the Portage Lake lots, and the park sends text alerts when the beach or parking areas close. Local health departments test Michigan public beaches during the season, and the health department has closed Sugarloaf Lake in this park to swimming before for high E. coli counts, so look up current results on the state's BeachGuard map before a visit. Big Portage has a public boat launch close to the beach, so expect boat traffic and stay inside the marked swim area.

JR's Concessions runs the store in the beach house, selling food, groceries and camping supplies, and rents watercraft there from mid-May through early October. The day-use area also has two reservable picnic pavilions, picnic tables and grills, a playground, an accessible fishing pier, an ADA-compliant boat launch and an 18-hole disc golf course that stays open all year. The Waterloo-Pinckney Trail has a trailhead at Portage Lake, and the rest of the park offers 47 miles of hiking trails, mountain bike and horse trails, more fishing lakes and the Gerald E. Eddy Discovery Center on Bush Road.