Flassavatnet is a lake in Rogaland county, Norway. The lake lies on the border of the municipalities of Gjesdal and Sandnes. The 0.9243 km2 lake sits about 1 km north of the large village of Ålgård, not far north of the lakes Edlandsvatnet and Limavatnet. The lake is a reservoir with a small dam on the southern edge of the lake. The water level is kept at about 177 m above sea level. The reservoir holds about 900000 m3.