Iberian gauge (Spanish: ancho ibérico, trocha ibérica, Portuguese: bitola ibérica) is the name given to the track gauge (distance between the two rails of a track) most extensively used by the railways of Spain and Portugal: namely 1,668 mm (5 ft 5 ²⁄₃ in).

As finally established in 1955, the Iberian gauge is a compromise between the similar, but slightly different, gauges adopted as respective national standards in Spain and Portugal in the mid-19th century. The main railway networks of Spain were initially constructed to a 1,672 mm (5 ft 5 ⁵⁄₆ in) gauge of six Castilian feet. Those of Portugal were instead built to a 1,435 mm (4 ft 8 ¹⁄₂ in) and later railways to a 1,664 mm (5 ft 5 ¹⁄₂ in) gauge of five Portuguese feet – close enough to allow interoperability with Spanish railways.

A commonly cited reason for the adoption of this gauge, broader than which became the international standard, is that it was intended to hinder any use of the railroad in any of French invasion attempts. This is possible, but the January 19, 1845 governmental decree does not state any reason, simply the gauge itself. It is known that the government entrusted such decisions to a committee, whose main work was done by civil engineers Juan Subercase and Calixto Santa Cruz: both good civil engineers but with little experience in railroads. Their judgement, made because of their profession and lack of experience in railroads, was that since the Spanish landscape was more mountainous than that of England and the France, a wider gauge would make more powerful locomotives possible, with larger boilers and, more importantly, larger fireboxes. This was obviously a miscalculation; in other areas of Spain, narrower gauges were used in the most mountainous areas, since Iberian gauge works in such areas was too expensive. One example of this is the 1,000 mm (3 ft 3 ³⁄₈ in) gauge FEVE.