TT scale (from "table top") is a model railroading scale at 1:120 scale with a track gauge of 12 mm between the rails. It is placed between HO scale (1:87) and N scale (1:160). Its original purpose, as the name suggests, was to make a train set small enough to assemble and operate on a tabletop.

The scale originated in the USA, but is today widespread mainly in Central Europe, thanks to Rokal and "Berliner-TT-Bahnen", defunct German manufacturers of train sets in TT. It is the second-most popular scale in Central Europe and Russia, after HO, with several manufacturers based in countries such as Germany and the Czech Republic, and was reintroduced to the United Kingdom in 2022. Adherents to the scale maintain it is the smallest practical scale, especially for those who like to build models from scratch.

In wargaming, TT scale equals the 15 mm scale where the height of "standard" 180 cm (5 ft 11 in) soldier height is 15 mm (0.59 in). For British 3 mm TT scale, see 3 mm scale.

TT scale was invented in the United States by Hal Joyce, a former automotive designer. He founded a company, H. P. Products, in 1945, and the first advertisement appeared in 1946. The product line included locomotive kits, passenger and freight car kits, track and detail parts. By the early 1950s, it had a following, offering less detail than HO—considered by some to be an advantage at the time—and a lower price than most other scales. Numerous other companies began offering TT scale trains, track, and accessories as well. In the immediate period afterward, several other American manufacturers also began production of TT items, notably the Kemtron Corporation (founded by Levon Kemalyan), which manufactured metal castings in several scales.

In Europe, Rokal and Zeuke (which became Berliner-TT-Bahnen and ultimately Tillig) began production in West and East Germany respectively towards the end of the 1950s. British Tri-ang Railways also introduced the scale to the UK, but due to the smaller British loading gauge, the trains were scaled up to 1:101.6 (or 3 mm scale) to have room for motors and other electric equipment. For a while the scale enjoyed considerable success, but it was not meant to last.

By the early 1960s, TT had been eclipsed in popularity by N scale, which is even smaller. Tri-ang quit the scale in 1967, and H. P. Products discontinued manufacturing their TT line in 1968. In West Germany, Rokal was sold to competitor Wesa (which marketed their own 13 mm-track, 1:100 scale), and production ended in 1969.

In East Germany, then on the other side of the Iron Curtain, production continued. Berliner-TT-Bahnen, which had superseded Zeuke, widened its offerings and exported trains and tracks over most of the Eastern Bloc. As the range became more and more comprehensive, offering not only trains and tracks but also accessories such as signals, lights and working catenaries, it became a serious contender to HO. During the 1980s, train sets were also exported to the West, offered as simple and cheap entry-models to the hobby and earning East Germany much-needed hard currency.

With the end of the Cold War, the East German economy was liberalised. Berliner-TT-Bahnen started to update their offerings, but soon ran into financial hardship. The company eventually ended up in the hands of Tillig, still today the largest provider of TT equipment. The products were improved, with better technology and more details, to bring them up to date with the offer in other scales.