Seaforth Island (Scottish Gaelic: Eilean Shìphoirt/Shìophoirt or Mulag) is an uninhabited island in the Outer Hebrides of Scotland.

Unlike many other islands of the Outer Hebrides which are mainly surrounded by open sea, Seaforth Island lies in a narrow fjord-like sea loch named Loch Seaforth, 8 kilometres (5.0 mi) from the open waters of The Minch.

The island has poor soil which only supports rough grazing.

There are no census records indicating inhabitation in the recent past, although the loch area was the subject of border disputes in the 19th century.

In 1851 these were resolved by the unusual decision to allocate the whole of Seaforth Island to both counties, Ross-shire and Inverness-shire, which at the time controlled Lewis and Harris respectively.