The Black-tailed Godwit is a large wader with long bill (7.5 to 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. The sexes are similar,[2] but in breeding plumage, they can be separated by the male's brighter, more extensive orange breast, neck and head. In winter, adult Black-tailed Godwits have a uniform brown-grey breast and upperparts (in contrast to the Bar-tailed Godwit's streaked back). Juveniles have a pale orange wash to the neck and breast.[3]

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.[3][4] Black-tailed Godwits are similar in body size and shape to Bar-taileds, but stand taller.[2]

It measures 42 cm from bill to tail with a wingspan of 70–82 cm.[2] Males weigh around 280 g and females 340 g.[5] The female is around 5 % larger than the male,[2] with a bill 12-15% longer.[6]