The Black-tailed Godwit is a large wader with long bill (which is 12 cm long), neck and legs. During the breeding season, the bill has a yellowish or orange-pink base and dark tip; the base is pink in winter. The legs are dark grey, brown or black.
In flight, its bold black and white wingbar and white rump can be seen readily. When on the ground it can be difficult to separate from the similar Bar-tailed Godwit, but the Black-tailed Godwit's longer, straighter bill and longer legs are diagnostic. Black-tailed Godwits are similar in body size and shape to Bar-taileds, but stand taller.
It measures 42 cm from bill to tail with a wingspan of 82 cm. Males weigh around 280 g and females 340 g. The female is larger than the male.