Demnia () is a village in the Trostianets rural hromada of the Stryi Raion of Lviv Oblast in Ukraine.
The first written mention of the village was in 1453.
In the Second Polish Republic, until 1934, the village constituted an independent rural municipality (gmina) within the à »ydaczów County of the Stanisà Âawów Voivodeship. As part of the consolidation reform, on August 1, 1934, it was incorporated into the newly established rural Mikoà Âajów nad Dniestrem in the same county and voivodeship.
In April 1944, Ukrainian nationalists from the OUN-UPA murdered 13 Poles here, while looting Polish households, part of Volhynia genocide.
After the war, the village became part of the administrative structures of the Soviet Union.
On 19 July 2020, as a result of the administrative-territorial reform and liquidation of the Mykolaiv Raion, the village became part of the Stryi Raion.