A whaler is a specialized ship, designed for whaling, the catching and/or processing of whales. The former included the whale catcher, a steam or diesel-driven vessel with a harpoon gun mounted at its bows. The latter included such vessels as the sail or steam-driven whaleship of the 16th to early 20th century and the floating factory or factory ship of the modern era. There were also vessels that combined the two, such as Bottlenose whalers of the late 19th and early 20th century and catcher/factory ships of the modern era.

Whaleships had two or more whaleboats, open rowing boats used in the capture of whales. Whaleboats brought the captured whales to the whaleships to be flensed or cut up. Here the blubber was rendered into oil using two or three try-pots set in a brick furnace called the tryworks.

At first, whale catchers either brought the whales they killed to a whaling station or factory ship anchored in a sheltered bay or inlet. Later, with the development of the stern slipway, whale catchers brought their catch to factory ships operating in the open sea.