The official TT website (www.iomtt.com) said 34-year-old Irish rider Derek Brien was killed after a high-speed accident during the first Supersport race.
“Derek was an experienced road racer who first competed on the Isle of Man in the Manx Grand Prix in 2007 and competed in the TT races in 2009 and 2010,” said a statement from the Auto Cycle Union (ACU).
Last week sidecar competitor Bill Currie, 67, and his 59-year-old passenger Kevin Morgan were killed during a qualifying session.
The TT course pits riders against the clock on public roads on the island with top speeds exceeding 200-mph (320-kph). There have been more than 220 deaths during practising or racing since the inaugural Tourist Trophy event of 1907.
Despite the perils of the twisting narrow roads and calls from critics for the TT to be banned, the races remain popular with competitors and spectators alike.