Andrew Sydney Withiel Thomas, AO (born 18 December 1951) is an Australian and American aerospace engineer and a former NASA astronaut.

He is the great-great-grandson of Frederick George Waterhouse, first curator of the South Australian Institute Museum, and naturalist of the John McDouall Stuart Expedition 1861–1862.

In August 1993, following one year of training, he was appointed a member of the NASA Astronaut Corps and was qualified for an assignment as a mission specialist on Space Shuttle flight crews.

NASA officially announced Thomas' retirement on 20 June 2014, which took effect on 1 March 2014, after 22 years with the space agency.

STS-77 was a mission during which the crew deployed two satellites, tested a large inflatable space structure in orbit and conducted a variety of scientific experiments in a Spacehab laboratory module carried in Endeavour's payload bay.

STS-114 Discovery (26 July – 9 August 2005) was the Return to Flight mission following the Columbia accident during which the crew continued the assembly of the International Space Station.

Thomas tested and evaluated new procedures for flight safety and inspection and repair techniques for the Shuttle's thermal protection system.

After a 2-week, 5.8 million mile journey in space, the orbiter Discovery and its crew returned to land at Edwards Air Force Base, California.