Foster is an unincorporated community partly within the city of Sweet Home in Linn County, Oregon, United States. It is located on the western end of the Foster Reservoir. A post office named Foster serves ZIP code 97345, which serves areas beyond the Sweet Home city limits on both the north and south sides of Foster Lake: on the north side, the area extends along the Middle Santiam River to the west end of Green Peter Lake; on the south, the area extends for several miles along the South Santiam River and Santiam Highway.This region experiences warm and dry summers, with no average monthly temperatures above 71.6 °F. According to the Köppen Climate Classification system, Foster has a warm-summer Mediterranean climate, abbreviated "Csb" on climate maps.