Beira Lake, Freshwater reservoir in central Colombo, Sri Lanka

Beira Lake is a man-made lake in central Colombo made up of several connected basins, including the larger East Lake and the smaller West Lake, with Slave Island and Union Place between them. It covers about 0.65 km2, down from roughly 1.65 km2 a century ago, and is linked to canals and spillways such as the St. Bastian Canal and Mc. Callum Gates.

The Portuguese began digging a moat around Colombo Fort in the early 16th century to defend against local kings. Captain Lopo de Brito found a stream between the mounts of Dematagoda and St. Bastian, and its water was diverted into the moat to form the lake. In 1587 Rajasingha I of Sitawaka cut several canals to drain the crocodile-infested lake but failed to defeat the Portuguese, who brought reinforcements by sea.

The Seema Malaka temple, part of the Gangaramaya monastery southeast of the lake, floats on the water and is reached by a walkway. The monk Hikkaduwe Sri Sumangala Nayaka Thera (1827–1911) founded Gangaramaya. The lake's name is recent: it first appeared on maps in 1927, before which it was simply called Colombo Lake.

The lake is open daily from 7 AM to 6 PM with no entry fees required. Swan boat rentals are available for those interested in exploring the water.

The lake's name only appeared on maps in 1927; before that it was called Colombo Lake. A granite plaque recovered from the lake's sluice reads "De Beer 1700", supporting the theory that it honours the Dutch engineer Johann de Beer, who built canals and drained surrounding marshland around 1700.