Formica exsecta (the narrow-headed ant or excised wood ant) is a species of ant found from Western Europe to Asia.

A rare formicine ant with a deeply excised head, F. exsecta forms small mounds up to around a foot in height consisting of much finer material than that used by "true" wood ants of the F. rufa group.

An interesting feature of F. exsecta is that it occurs in two distinct social forms: either a monogyne form where the colony has a single egg-laying queen, or a polygyne form where many egg-laying queens are part of the same colony.

F. exsecta is placed in the Coptoformica subgenus within the genus and is closely related to Formica exsectoides, an American species. Both species may form vast colony networks. The largest known polydomous system of F. exsecta consists of 3,350 nests dispersed over about 22 ha in Transylvania, Romania.

In Great Britain, F. exsecta can be found only in a few scattered heathland locations in South West England — principally Chudleigh Knighton Heath and nearby Bovey Heath, which are both managed by the Devon Wildlife Trust, and in the central Scottish Highlands (including Rannoch Moor). A population centre previously existed in the New Forest, and such eminent myrmecologists as Horace Donisthorpe recorded this species there and in Parkhurst Forest on the Isle of Wight in the last century, but this seems to have declined considerably over the past few decades, and recent searches in such locations have failed to find any trace of colonies. The narrow-headed ant is currently one of the target species in the Back from the Brink project, which aims to extend its range in England.

F. exsecta has also been found in forests in Sweden, Finland, Germany, Tibet, and China.