A heat exchanger, a heat exchanger unit, and a refrigeration cycle apparatus in which frost melt water is inhibited from reaching an upper surface of a header include: heat transfer tubes arranged in parallel with each other; a fin connected to one of the heat transfer tubes; and a header connected to the heat transfer tubes and having a header end surface along a direction in which the heat transfer tubes are arranged in parallel. The fin has an edge facing the header and extends in a first direction perpendicular to the axes of the heat transfer tubes. An end portion of the fin projects in the first direction relative to the header end surface, and another end portion of the fin in the first direction is positioned closer in the first direction to the heat transfer tubes than the header end surface is.