Lock and Dam Number 52, Lock and dam complex on the Ohio River, Illinois, United States

Lock and Dam Number 52 was a navigation complex on the Ohio River, located near Olmsted, Illinois, close to the Kentucky border. It had two separate locks of different widths to accommodate the range of commercial vessels using the river.

The U.S. Army Corps of Engineers built the complex in 1929 as part of a broader effort to make the Ohio River reliably navigable for commercial traffic. After nearly a century of use, it was torn down between 2019 and 2020 once a replacement facility had come into service.

The complex was demolished in 2019–20, so no lock structures remain at the site. The name Lock and Dam 52 refers to its position as the 19th lock and dam on the Ohio River, 939 miles downstream of Pittsburgh and 23 miles upstream from the Mississippi confluence. The nearby Olmsted Lock and Dam, completed in 2018, replaced it.

Since the complex has been demolished, there are no standing structures to see at the site today. Visitors interested in the history of river navigation in the area can still view the Ohio River from the surrounding banks to get a sense of the waterway's scale and importance.

In 2015, 80.2 million tonnes of cargo passed through Lock and Dam 52, which the New York Times called the busiest and most economically important lock in the United States. The same article described it as a "serious bottleneck", with delays of 15 to 20 hours, and reported that replacing it late cost $640 million a year.