Dafydd Rhys "Dave" Williams (born May 16, 1954) is a Canadian physician and a retired CSA astronaut. He had two spaceflights, both of which were Space Shuttle missions. His first spaceflight, STS-90 in 1998, was a 16-day mission aboard Space Shuttle Columbia dedicated to neuroscience research. His second flight, STS-118 in August 2007, was flown by Space Shuttle Endeavour to the International Space Station. During that mission he performed three spacewalks, becoming the third Canadian to perform a spacewalk and setting a Canadian record for total number of spacewalks. These spacewalks combined for a total duration of 17 hours and 47 minutes.

In 1998, Williams became the first non-American to hold a senior management position within NASA, when he held the position of Director of the Space and Life Sciences Directorate at the Johnson Space Center.

He attended high school in arber Beaconsfield, Quebec and earned a B.Sc in from McGill University, Montreal, in 1976, a master of science degree in physiology, MD, and master of surgery degrees from McGill University, Montreal, in 1983. He completed a residency in family practice in the faculty of medicine, University of Ottawa, in 1985 and obtained fellowship in emergency medicine from the Royal College of Physicians and Surgeons of Canada, following completion of a residency in emergency medicine at the University of Toronto, in 1988.