Const. Andrew Curtis says the mask was stolen last Friday from someone's shed in Alert Bay on Cormorant Island on the Inside Passage between Vancouver Island and the mainland.
Curtis says the art piece depicts the supernatural bird Huxhukw (HOO-hoowk), and is one of three bird masks used by the Kwakwaka'wakw (KWAK-wak-ya-wak) peoples during winter ceremonies.
The stolen mask has a five-foot-long beak, and is painted in bright colours.
Police believe whoever has the mask now may try to sell it to a private collector or a gallery, and Curtis is urging anyone with information to contact the RCMP.
Use of aboriginal masks was banned in Canada from the late 1800s until 1951, and during that time many masks were seized. While some were returned to First Nations, others are now held in public and private collections.