The head, neck, breast, belly, and underwings are primarily white, and the legs are pink.

In summer, the head and neck are white, with the bill turning bright yellow with a larger red spot on the lower mandible.

These gulls will lay 3 bluish or greenish eggs in nests lined with grass, moss or lichens.

After numerous papers had been written suggesting downgrading this species to a subspecies or even a morph of Iceland gull, the American Ornithologists' Union invalidated Thayer's gull as a full species in the 2017 annual supplement to the American Ornithologists' Union checklist.

[8] Along with the AOS, the BOU lumps the three (thayeri, kumlieni, and glaucoides) as forms of Iceland gull.