Cull Canyon Swim Complex is a fenced, lifeguarded swim lagoon inside Cull Canyon Regional Recreation Area, a 360-acre East Bay Regional Park District park on the north side of Castro Valley in Alameda County. The lagoon covers about 1.5 acres and sits in the bottom of Cull Canyon beside Cull Creek. When the district built it in the 1960s, it put in a secondary dam so the swim area would hold its own water level instead of rising and falling with the flood control reservoir next door. That design won a 1966 Governor's Design Award for landscape. The old reservoir to the south has since silted in and is largely gone, but the lagoon was kept separate and still operates.

The park is a short drive north of Interstate 580. Heading east on I-580, take the Grove Way exit, go north on Grove Way, then turn left onto Cull Canyon Road; the entrance is on the left. The swim complex address is 18627 Cull Canyon Road. Parking is free, and the lot is open 8am to 10pm from March through November and 8am to 8pm in winter. There are also walk-in entrances off Heyer Avenue and Columbia Drive for people coming on foot from the surrounding neighborhood.

Inside the gate you get a white sand beach that slopes into a chlorinated lagoon, with a long run of shallow water that suits small children and a bathhouse and vending machines near the entrance. Because the water is treated and held behind its own dam, the level stays put through summer instead of dropping the way the old reservoir did. Swimming is allowed only while lifeguards are on duty. Free beach wheelchairs are lent on a first-come basis, and a few picnic tables and charcoal grills inside the fence are open to anyone who gets there first.

Admission is a district day-use fee, not free. Adults 18 to 61 pay $5, ages 2 to 17 and seniors 62 and over or disabled visitors pay $3, children under 2 get in free, and pre-registering online for a weekend or holiday slot costs $6 per person. The kiosk is cashless (Visa, MasterCard, Discover or tap to pay). In 2026 the complex opened weekends and holidays from May 16 to September 27, from 11am to 6pm with the water closing at 5:30pm. The district now says it will reopen for the 2027 season, so check its swimming page for next year's dates before you go. Fees are subject to change.

The biggest practical issue is crowding. On hot weekends and holidays the complex reaches capacity early, and once it does, no one else gets in for the rest of the day, so pre-register or come at opening. Dogs are not allowed in the swim area. Alcohol, glass containers and portable grills or stoves are banned inside the fence, and fire restrictions may apply in dry months. Outside the season, and any time lifeguards are off duty, the lagoon is closed to swimming.

Outside the swim complex the park has reservable picnic areas (groups of 30 or more must book), lawns, and a section of the Chabot-to-Garin Regional Trail, part of the Bay Area Ridge Trail. The trail follows Cull Creek and climbs north about six miles to EBMUD's Chabot Staging Area on Redwood Road; the stretch through East Bay Municipal Utility District land requires an EBMUD trail permit. Dogs are welcome on the trails.