This species is a somewhat plump wader with a dark greenish-brown back and wings, greyish head and breast and otherwise white underparts. The back is spotted white to varying extents, being maximal in the breeding adult, and less in winter and young birds. The legs and short bill are both dark green.
It is conspicuous and characteristically patterned in flight, with the wings dark above and below and a brilliant white rump. The latter feature reliably distinguishes it from the slightly smaller but otherwise very similar Solitary Sandpiper (T. solitaria) of North America.
In flight it has a characteristic three-note whistle.