Trainer is a borough in Delaware County, Pennsylvania, United States. The population was 1,828 at the 2010 census, down from 1,901 at the 2000 census. The borough was named after David Trainer, a wealthy textile manufacturer.

Delaware County, colloquially referred to as Delco, is a county in the Commonwealth of Pennsylvania. With a population of 576,830 as of the 2020 census, it is the fifth-most populous county in Pennsylvania and the third-smallest in area. The county was created on September 26, 1789, from part of Chester County and named for the Delaware River. The county is part of the Southeast Pennsylvania region of the state.