The 1925 Isle of Man Tourist Trophy was the second and final year of the Ultra-Lightweight class for motorcycles of 175 cc capacity. This was the third year of the Sidecar race, which was also dropped after 1925.
After numerous retirements in 1924, Wal Handley won the Junior TT race over six laps of the Mountain Course on a Rex-Acme motorcycle at an average speed of 65.02 mph (104.64 km/h). Later in the week, Handley became the first TT rider to win two races in a week when he won the four-lap Ultra-Lightweight TT race, again on a Rex-Acme, setting a race record average speed of 53.45 mph (86.02 km/h), and a new lap record of 41 minutes, 52 seconds at an average speed of 54.12 mph (87.10 km/h).
During the Lightweight TT race, Wal Handley led the first two laps by over two minutes from C. W. "Paddy" Johnston, riding a Cotton, but a puncture caused Handley to slip off his motorcycle at Signpost Corner. The race was eventually won by Eddie Twemlow on a New Imperial at an average speed of 57.74 mph (92.92 km/h) from Johnston and Eddie's brother, Ken Twemlow, riding a New Imperial. The Senior TT race was sensationally won by Howard Davis while competing against the works teams with a motorcycle of his own manufacture, an HRD, in 3 hours, 25 minutes, 8 seconds at an average speed of 66.13 mph (106.43 km/h). A new lap record was posted by Jimmie Simpson, in 32 minutes and 50 seconds and an average speed of 68.97 mph (111.00 km/h) on an AJS motorcycle, but he failed to finish.