Croglio is a former municipality in the district of Lugano in the canton of Ticino in Switzerland . On 18 April 2021 the municipalities of Croglio, Monteggio , Ponte Tresa and Sessa merged to form Tresa.

Croglio is first mentioned in 1335 as burgus de Crolio.

Croglio had an area, as of 1997, of 4.38 square kilometers (1.69 sq mi). Of this area, 0.89 km2 (0.34 sq mi) or 20.3% is used for agricultural purposes, while 3.09 km2 (1.19 sq mi) or 70.5% is forested. Of the rest of the land, 0.63 km2 (0.24 sq mi) or 14.4% is settled (buildings or roads), 0.02 km2 (4.9 acres) or 0.5% is either rivers or lakes and 0.05 km2 (12 acres) or 1.1% is unproductive land.