The Gadwall is 46–56 cm (18–22 in) long with a 78–90 cm (31–35 in) wingspan. The male is slightly larger than the female, weighing on average 990 g (35 oz) against her 850 g (30 oz). The breeding male is patterned grey, with a black rear end, light chestnut wings, and a brilliant white speculum, obvious in flight or at rest. In non-breeding (eclipse) plumage, the drake looks more like the female, but retains the male wing pattern, and is usually greyer above and has less orange on the bill.
The female is light brown, with plumage much like a female Mallard. It can be distinguished from that species by the dark orange-edged bill, smaller size, the white speculum, and white belly. Both sexes go through two moults annually, following a juvenile moult.
The Gadwall is a quieter duck, except during its courtship display. Females give a call similar to the quack of a female Mallard but higher-pitched, transcribed as gag-ag-ag-ag. Males give a grunt, transcribed as nheck, and a whistle.