The Pyrenean Mastiff or Mastín del Pirineo is a Spanish breed of large livestock guardian dog from the autonomous community of Aragón in north-eastern Spain.

The Pyrenean Mastiff originated in the historic Kingdom of Aragon, where it has been documented since the Middle Ages.

[1]: 553  Its origins and purpose were in the annual transhumance of flocks to the high pastures of the Pyrenees for the summer months, and the return to lower ground for the winter.

Like other European flock guardian breeds, they were often fitted with a spiked metal wolf collar or carlanca for extra protection.

[1]: 553 In the latter 1940s the wolf disappeared from the Pyrenees, and once it was no longer needed for its principal function, the breed began to decline.