Turbo-Hydramatic 425 (TH425 or THM 425,[1] later 325) was an automatic transmission developed and produced by General Motors.

The internal parts spin the opposite direction in the THM425; the helical angle of the planetary gears is "reversed" and the one-way clutches freewheel in the opposite direction, for example.

The THM425 was developed for the 1966 Oldsmobile Toronado and the 1967 Cadillac Eldorado.

A lighter-duty transmission known as the THM325 (using components sourced from the THM200) replaced the THM425 in both car lines after the 1978 model year.

The THM325 became the THM325-4L with an overdrive added in 1982, but all vehicles using this transmission switched to more-conventional transverse engine mounting in 1986.