Øyeren is a lake in the Glomma River watershed, southeast of the city of Lillestrøm. It is located within the municipalities of Enebakk, Lillestrøm, and Rælingen in Akershus county and Indre Østfold municipality in Østfold county.

Lake Øyeren is the ninth largest lake by area in Norway with a surface area of 84.7 square kilometers (32.7 square miles). It is 101 meters (331 feet) above sea level and 76 meters (249 feet) deep.

The name of the lake (Norse Øyir) is derived from øy f 'island; flat and fertile land along a waterside'.

An area that includes parts of the northern ("nordre") end of the lake is an established nature preserve, listed as a Ramsar site.

• Øyeren information center (Norwegian )
• Nordre Øyeren natur preserve (Norwegian )
• Ramsar Sites Information on Nordre Øyeren (English )