Birgittas udde, Medieval castle ruins at Lake Boren, Sweden.

Birgittas udde is a medieval castle ruin on a headland jutting into Lake Boren. Stone house foundations, a preserved basement structure, and the trace of dried moats mark where buildings once stood near the water.

The manor on the headland was probably built in the 13th century; the earliest known owner is Ulf Gudmarsson. Tradition holds that the stronghold was abandoned after his death at Alvastra Abbey in 1344, when Birgitta moved to a new farm, Gamlegården, by present-day Brittås to the south.

The headland was long called S:a Britas näs, and the form Brittnäs still appears on topographic maps. The name comes from Birgitta, who lived on the farm for about twenty years in the 1300s. Tradition holds that after her husband Ulf Gudmarsson died at Alvastra Abbey in 1344, she moved to a new farm, Gamlegården, near today's Brittås to the south.

The site lies on a headland on the south shore of Lake Boren, in Ekebyborna parish, Motala Municipality. The ruins are reached on foot; the headland was once bounded on the landward side by two moats, and the stone building's ruin is what remains. The site has not been fully excavated, so stay on existing paths.

Large holes on both sides of the basement doorway show where a protective bar once held to keep intruders out. This detail preserves an everyday medieval security system still visible today.