Rex, Rex Motorcycles, Rex-Acme, (not to be confused with the German manufacturer of similar name) was a car and motorcycle company which began in Birmingham, England in 1900. Rex soon merged with a Coventry maker of bicycles and cars named Allard and then later in 1922 the company merged with Coventry's 'Acme' motorcycle company forming 'Rex Acme'. The company existed until 1933, and, in its heyday, was considered one of the greatest names in the British motorcycle industry.

William Williamson formed the Birmingham Motor Manufacturing and Supply Co in mid-1901. William was described as having entered the motor trade a few years before and as being well known in racing circles, and was an ex-winner of the Catford Hill-Climb (on a bicycle). The firm used the Rex trademark for their light car, and for their 1.75 hp Rex motorcycle. They had their registered office at 189 Broad Street, Birmingham, but their works was in Coventry.

In March 1902 the assets and goodwill of the company were bought by Allard & Co, founded in 1889 by Frederick W. Allard and George Pilkington, who had made a motor tricycle and a light car but had originally manufactured bicycles. In June 1902 they changed their name to the Rex Motor Manufacturing Co. Ltd. of Osborne Road, Earlsdon, Coventry. They enlarged the factory, and produced a range of three and four wheel cars and motorcycles. The founders fell out with the board and left the company in 1911.

In 1919 Rex Motor Manufacturing Co. merged with Coventry Acme, and by 1921 the two companies were selling Rex-Acme motorcycles. Wal Handley then raced Rex-Acme motorcycles, making them famous, and even became a company director, but left in 1928 to ride different machinery. Handley rode Blackburne 173 cc singles, and, in 1926, ohv 498 cc V twins. The name began to fade, and in 1932 was taken over by side-car manufacturers, Mills-Fullford, who ceased manufacture of Rex-Acme motorcycles in 1933. Other famous motorcycle racers that rode Rex-Acme motorcycles were H. G. Tyrell Smith, Arthur Taylor, Charles Needham, Hans Hasenauer, Felice Bonetto, Karl Machu and Otto Cecconi.

The "Rex King of British Motor Cars" was exhibited on Stand 20 at the 1901 Crystal Palace motor show. This was a 6.5HP 4-seater, but over the next year there were successive power increases and by the Spring 1903 exhibition at the Agricultural Halls the car was available with up to 16HP. Their display also showed the 10HP engine, a water-cooled single of their own making with automatic inlet valve.

In May 1904 the Rexette arrived in the showrooms, marketed as the "King of Little Cars", it was a 5HP tricar with tandem seating with the driver at the back powered through a single rear wheel.

The Rex Motorcycle was advertised in 1902 as King of Motor Bicycles, challenges any Motor Bicycle in the World of whatever HP weighing under 100 lbs for speed, hill-climbing and economy.

Around 1903 Rex produced motorcycles with engines above and driving, the front wheel. (the Werner engine position). The engines were Rex’s own, and included singles and V twins. In 1904 there was a 372 cc model which had the silencer cast with the cylinder in one piece. In 1905 the side valve Rex model was rated at 3.25 hp. In 1907 they introduced the Rex Lightweight with a new frame design, the engine was 76mm bore by 80mm stroke (362cc), with automatic inlet valve, and direct belt drive. At the 1908 Stanley Show both 3.5HP single and 5HP twin motorcycles were on show (both with automatic inlet valves), the 5HP with 2-speed gear. For the 1909 show they introduced a 2-stroke with the same bore and stroke as their single, and a pair of 'Speed King' motorcycles, versions of their single and V-twins fitted with rigid frame, unsprung forks, and direct belt drive.