The Hog's Back is a hilly ridge, part of the North Downs in Surrey, England. It runs between Farnham in the west and Guildford in the east.

Compared with the main part of the Downs to the east of it, it is a narrow elongated ridge, hence its name.

Jane Austen, in a letter to her sister Cassandra dated Thursday 20 May 1813 from her brother's house in Sloane Street, wrote of her journey to London in a curricle via "the Hog's-back"

This shows that it was known as the Hog's Back by Jane Austen's time. However, the medieval name for the ridge was Guildown (recorded first in 1035 where it was the site of the abduction of Prince Alfred of Wessex by Earl Godwin and then in the Pipe Rolls for 1190 and onwards) but this name is no longer in use. However, the name Guildown is evoked by Guildown Road, a residential road that climbs the southern side of the ridge on the southwestern fringes of Guildford.

The Guild- element of Guildown is the same as that found in Guildford, meaning "gold". Various explanations have been suggested for the relationship between the names of Guildown and Guildford. Guildown may be an abbreviation of Guildford Down ("the Down by Guildford"). The Hog's Back ends at Guildford, which is the point where the River Wey cuts through the North Downs. Alternatively, both Guildown and Guildford may derive independently from a gold-coloured feature; either the yellow flowers of the marsh marigold or the gold-coloured (sandy) soil of the hillside.

The Hog's Back is formed of chalk of Cretaceous age, laid down in shallow seas from the deposition of the calcium carbonate skeletons of micro-organisms. The Hog's Back is the surface representation of a monoclinal fold, where once horizontal chalk beds have been folded into steeply sloping layers. Across much of the North Downs, the angle of the chalk as it disappears below the surface is relatively shallow (c. 18° near Dorking), however at the Hog's Back, local faulting has produced a much steeper angle (up to 55°). To the north of the ridge lies the London Clay, and to the south the clays of the Wealden Group. The chalk is more resistant to weathering than the flanking clays, leading to the ridge's prominence over the surrounding terrain.

The Hog's Back gives its name to the geomorphological landform known as a Hogback, which is a long narrow ridge or series of hills with a narrow crest and steep slopes of nearly equal inclination on both flanks.

The Hog's Back has a minimum prominence (drop) of 55 metres (180 ft), reaching 154 m (505 ft) above sea level, the 24th highest hill in Surrey.