Competing for Great Britain at the 2012 Summer Olympics, he won a silver medal behind his arch-rival Dániel Gyurta in the 200-metre breaststroke. He broke the British record three times in the process, twice in the qualifying heats and once in the final (2:07.43), and came to within 0.12 seconds of the previous world record set by Christian Sprenger in 2009 (2:07.31). Only a new world record by two-time world 200 m breaststroke champion Dániel Gyurta denied Jamieson the gold.