A hub gear, internal-gear hub, internally geared hub or just gear hub is a gear ratio changing system commonly used on bicycles that is implemented with planetary or epicyclic gears. The gears and lubricants are sealed within the shell of the hub gear, in contrast with derailleur gears where the gears and mechanism are exposed to the elements. Changing the gear ratio was traditionally accomplished by a shift lever connected to the hub with a Bowden cable, and twist-grip style shifters have become common.

Hub gear systems generally have a long and largely maintenance-free life though some are not suitable for high-stress use in competitions or hilly, off-road conditions. Many commuter or urban cycles such as European city bikes are now commonly fitted with 7-speed gear-hubs and 8-speed systems are becoming increasingly available. Older or less costly utility bicycles often use 3-speed gear-hubs, such as in bicycle sharing systems. Many folding bicycles use 3-speed gear-hubs. Modern developments with up to 18 gear ratios are available.

Before epicyclic gears were used in bicycle hubs, they were used on tricycles. Patents for epicyclic hubs date from the mid-1880s. The first patent for a compact epicyclic hub gear was granted in 1895 to the American machinist Seward Thomas Johnson of Noblesville, Indiana, U.S.A. This was a 2-speed but was not commercially successful.

In 1896, William Reilly of Salford, England patented a 2-speed hub which went into production in 1898 as 'The Hub'. It was a great success, remaining in production for a decade. It rapidly established the practicality of compact epicyclic hub gears.

By 1902, Reilly had designed a 3-speed hub gear. He parted company with the manufacturer of 'The Hub' but had signed away to them the intellectual rights to his future gear designs. To circumvent this problem, the patents for Reilly's 3-speed were obtained in the name of his colleague, James Archer. Meanwhile, well-known English journalist and inventor Henry Sturmey had also invented a 3-speed hub. In 1903, Frank Bowden, head of the Raleigh Bicycle Company, formed The Three-Speed Gear Syndicate, having obtained the rights to both the Reilly/Archer and Sturmey 3-speeds. Reilly's hub went into production as the first Sturmey Archer 3-speed.

In 1902, Mikael Pedersen (who also produced the Dursley Pedersen bicycle) patented a 3-speed hub gear and this was produced in 1903. This was said to be based on the "counter shaft" principle but was arguably an unusual epicyclic gear, in which a second sun was used in place of a ring gear. In 1904 the Fichtel & Sachs company (Germany, Schweinfurt) produced a hub gear under license to Wanderer, and by 1909, there were 14 different 3-speed hub gears on the British market.

By the 1930s, hub gears were used on bicycles all over the world. They were particularly popular in the UK, the Netherlands, the German-speaking countries and Scandinavia. Since the 1970s, they have become much less common in English-speaking countries, but in many parts of northern Europe, where bicycles are regularly used as daily transport rather than merely for sport or leisure, hub gears are still widely used. The cheaper and stronger (but less reliable) derailleur systems, which offer a wider gear range, have now started to appear.

By 1987, Sturmey-Archer made only 3- and 5-speed hubs, and Fichtel & Sachs and Shimano made only 2- and 3-speed hubs. In that year, the first book (apart from service manuals) for some 80 years dealing solely with epicyclic bicycle gears was published. Since then, there has been a slow but steady increase in interest in hub gears, reflected in the wider range of products now available.