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 1925 Junior TT Race over six laps of the Mountain Course for Rex-Acme motorcycles at an average speed of 65.02 mph (104.64 km/h). Later in the week, Wal 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 motorcycle, setting a race record at an average speed of 53.45 mph (86.02 km/h) and a new lap record of 41 minutes, 52 seconds and an average speed of 54.12 mph (87.10 km/h). During the 1925 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 1925 Lightweight TT Race was eventually won by Eddie Twemlow on a New Imperial motorcycle at an average speed of 57.74 mph (92.92 km/h) from "Paddy" Johnston and Eddie's brother, Ken Twemlow, riding a New Imperial. The 1925 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 again 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 again, he retired from the 1925 Senior TT Race.