The Diamond DA40 Diamond Star is an Austrian four-seat, single engine, light aircraft constructed from composite materials. Built in both Austria and Canada, it was developed as a four-seat version of the earlier DA20 by Diamond Aircraft Industries.

Based on the success of the earlier DA20 two-seat aircraft, the company designed a four-seat variant, the DA40. The DA40 is a four-seat low-wing cantilever monoplane made from composite materials. It has a fixed tricycle landing gear and a T-tail. The Rotax 914-powered prototype DA40-V1, registered OE-VPC, first flew on the 5 November 1997 and was followed by a second prototype DA40-V2 (registered OE-VPE) which was powered by a Continental IO-240. In 1998 a third prototype DA40-V3 flew powered by a Lycoming IO-360 engine.

The DA40 has officially appeared in only three versions, the DA 40, DA 40D and DA 40F, as documented on its type certificates. The various model names that the aircraft has been sold under are marketing names and are not officially recognized by the authorities that have certified the aircraft.