The Alvis Three Litre TD21 was a British sports saloon or coupé made by Alvis cars between the end of 1958 and October 1963. It was a revised version of the TC 108G, the body was made by Park Ward who were better able to supply them to the quantity, quality, and price required. The cars were slightly taller and a drophead coupé was added to the range, they were both lighter.

The 2993 cc engine was again uprated, now producing 115 bhp (86 kW) mainly by an improved cylinder head and increasing the compression ratio from 8.0:1 to 8.5:1. A new four speed gearbox from the Austin-Healey appeared and Borg Warner three speed automatic transmission was offered. Overdrive was available on the manual transmission from late 1960 to 1962. Suspension was similar to the TC 21, independent at the front using coil springs with leaf springs at the rear but the track was increased by 1 in (25 mm) to 55.5 in (1,410 mm)[3] and a front anti roll bar added. Wire spoked wheels became an option. From 1959 the all drum brake set up was changed to discs at the front retaining drums at the rear.

A car with manual transmission was tested by the British magazine The Motor in 1960 and had a top speed of 103 mph (166 km/h) and could accelerate from 0-60 mph (97 km/h) in 13.5 seconds. A fuel consumption of 20.2 miles per imperial gallon (14.0 L/100 km; 16.8 mpg‑US) was recorded. The test car cost £2827 including taxes.