Wyndham Halswelle (May 30, 1882 – March 31, 1915) was a Scottish athlete, winner of the controversial 400 m run at the 1908 Summer Olympics.

Born in London of Scots parents, Wyndham Halswelle had a notable athletic career at Charterhouse School and the RC Sandhurst, before being commissioned into the Highland Light Infantry. Halswelle's ability was recognised while the regiment was in South Africa, where he participated in the Boer War, but it was not until he returned to Britain in 1904 that he took up athletics seriously.

In 1905 he won the Scottish and AAA 440 yd (402 m) titles, and a year later, in the Athens Intercalated Olympics, he achieved a silver medal in the 400 m and a bronze in the 800 m. On his return to Scotland he came first in the 100, 220, 440 and 880 yd (91, 201, 402, 805 m) - all on the same afternoon - at the Scottish championships. His season was cut short by a leg injury in 1907, but he came back the following year to set a world record of 31.2 for 300 yd (274 m).

He reached the Olympic final in 1908 with the fastest qualifying time, but in the final as four runners came into the final stretch, William Robbins (USA) was first, followed by John Carpenter (USA), with Wyndham Halswelle coming in third, followed by John Taylor (USA). As Carpenter and Halswelle swung out to pass Robbins, someone shouted "Foul!". Though Carpenter finished first, with Robbins in second and Halswelle in third, the British officials accused Carpenter of blocking Halswelle and voided the whole race. Picture evidence of the race indeed indicates Carpenter blocked Halswelle. While blocking was allowed under US rules at the time, the Olympic race was conducted under stricter, British rules. The race was ordered to be rerun in lanes without Carpenter, but since the American runners refused to redo the race, Halswelle ran the race all by himself to win the gold. It is the only occasion in Olympic history where the final was a walk-over. As a result of the controversy, from the next Olympics in 1912 onwards all 400 meter races were run in lanes, and the International Amateur Athletic Federation was founded to establish uniform worldwide rules for athletics.

Wyndham Halswelle made a farewell appearance at the 1908 Glasgow Rangers Sports and never ran again. Captain Wyndham Halswelle was killed by a sniper's bullet at the Battle of Neuve Chappelle in France, during World War I.