Marlin fishing or billfishing is offshore saltwater game fishing targeting several species of fast-swimming pelagic predatory fish with elongated rostrum collectively known as billfish, which include those from the families Istiophoridae and Xiphiidae (swordfish). It is considered by some fishermen to be a pinnacle of big-game fishing, due to the size, speed and power of the billfish and their relative elusiveness.

Marlins are fish from the family Istiophoridae, which includes 11 species.