The BMW 1 Series (E81/E87 and E82/E88 models) is a small family car produced by the German automaker BMW since 2004. The 1 Series is unusual in its class as it features rear-wheel drive, 50:50 weight balance, a longitudinally-mounted engine and an advanced aluminum multilink suspension. It is currently BMW's second best-selling automobile worldwide, accounting for nearly one-fifth of the total sales in 2008.

The 1 Series was launched globally in Autumn 2004 and shares many structural, chassis, powertrain, hardware and electronic elements with the larger 3 Series. The model was started to provide a lower point of entry into the BMW range as the 3 Series moved gradually up-market. Initially launched as a five-door hatchback, a three-door version was also launched in July 2007. The 1 Series is priced between the MINI and the current E90 3 Series. Because the coupe is the only rear wheel drive vehicle in its class, it is often considered^[who?] the successor to the BMW 2002.

The 1 Series coupe (E82) and convertible (E88) went on sale in the United States and Canada in model year 2008 (30 June 2007) as the 128i and the 135i. Other countries received the 120i and 125i in both platforms. The convertible, unlike the 3 Series convertible, uses a soft-top instead of a folding hardtop. This is lighter, cheaper to manufacture and preserves more trunk space than the folding hardtop would allow.