Lake Ronkonkoma is a census-designated place (CDP) in Suffolk County, New York, United States. The population was 20,155 at the 2010 census.
Lake Ronkonkoma is mainly located in the Town of Brookhaven, but has small sections in the Town of Smithtown and the Town of Islip.
Sachem High School North is located in Lake Ronkonkoma.
Lake Ronkonkoma is located at 40°49′47″N 73°6′47″W (40.829709, -73.113015).
According to the United States Census Bureau, the CDP has a total area of 4.9 square miles (13 km2), all land.
The actual lake, Lake Ronkonkoma, adjacent to the CDP, is the largest lake on Long Island. The land surrounding one side of the lake is in the jurisdiction of Islip. The elevation of the lake surface is given as 55 feet (17 m) on the most recent USGS map, but as the lake is a "groundwater lake", not fed by streams, it has no surface outlet and its water surface reflects the current level of the local water table. This can undergo significant changes over time, and the lake level experiences slow periods of rise and fall. In the late 1960s it was quite low; after several intermediate changes in level, in 2007 the lake was higher than at any time since, with a difference of well over 5 feet (1.5 m) between the 1960s low and the 2007 high.
As a result of the lake's existence, Lake Ronkonkoma was once a resort town, until the area experienced a population explosion in the mid-20th century.