Pico Tres Mares, Mountain summit in Cantabria, Spain.

Pico Tres Mares rises to 2,171 m (2,172.6 m per the Cantabrian government) on the border of Cantabria and Palencia, in the Sierra de Híjar. Its crest is exposed Triassic quartzite conglomerate, a broken wall of grey and brown rock with snow filling the gullies and ledges. The summit is the vertex where the Sierra de Híjar meets the Sierra del Cordel above the Circo de Tres Mares.

The name Tres Mares was coined by the Cantabrian writer Ángel de los Ríos, who first used it in verses dated 1849. Earlier documents called the peak Peña Labra, as the hydrologist Pedro Antonio de Mesa did in 1862 and Amalio Maestre in his 1864 geological memoir of Santander province; older local names were Pico Negro, Cueto Negro and Pico de los Asnos. A 1920 act described the stone that marked the summit, later cut down during works on the viewpoint.

The name was given by Ángel de los Ríos, who wrote in 1849 that the peak ends the Ebro valley "like a sharpened knife" and begins those of the Pisuerga and Nansa, bound for the Mediterranean, the Atlantic and the Cantabrian. Earlier names were Pico Negro, Cueto Negro and Pico de los Asnos. A small brick lookout and an iron cross stand on the summit, the cross a copy of an earlier one damaged by lightning.

A road passable by car reaches La Fuente del Chivo at 2,040 m, with a car park at its end, and the Alto Campoo ski station lies just below along the road. From there the summit is a short climb; a small brick lookout and an iron cross stand on or near the top, the cross a copy of an earlier one damaged by lightning.

The summit stone that once marked the peak was cut down during the many works and modifications of the existing viewpoint, as recorded in a 1920 jurisdictional act of the IGC. The peak also divides three historical jurisdictions: the Polaciones valley, the Hermandad de Campoo de Suso and Aguilar de Campoo.