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The S-Series is a Straight-4 SOHC internal combustion engine developed by the Austin Rover Group (subsidiary of British Leyland), and introduced in 1984 in both the Austin Montego and the Mark 1 Rover 200-series. It was also used in the Austin Maestro from 1985 onwards.

The engine comes from the same lineage as the BMC-developed E-Series family introduced in the 1969 Austin Maxi, but with important modifications in order to facilitate compatibility with a contentional "end-on" transmission unit, in place of BMC/BL's traditional "gearbox-in-sump" configuration for its front wheel drive vehicles. The gearbox flange was redesigned to accept either a Volkswagen manual gearbox (for the Maestro/Montego), or the Honda PG-1 transmission (for the Rover 200).

BL had also developed the earlier R-Series engine from the E-Series family, but largely as a stopgap at the Maestro's launch since the S-Series was not yet ready for production. Because the E-Series had to be turned through 180 degrees in order to facilitate an end-on transmission, the resultant R-Series unit had the inlet manifold on the front facing side of the cylinder head, something which proved fatal for the engine's reliability - since it opened the door for carburettor icing. The S-Series solved this problem, as the inlet manifold was now on the rear face of the engine. Another important advance over the E-/R-Series was the adoption of a toothed belt driven camshaft in place of the timing chain.

Production of the S-Series engine continued until the end of Montego/Maestro series production in 1993, the remaining cars which were built by Rover through until 1994 used only the O-Series diesel unit.