Measuring around 90 cm (35.5 in), the yellow-billed spoonbill has all white plumage. The long spoon-shaped bill, bare-skinned face, legs and feet are all yellow, while the iris is pale yellow.[4] The sexes are similar in plumage and coloration. In the breeding season, the face is lined with black, long hackles develop on the chest, and the wings have black tips.[5] The bill of the yellow-billed spoonbill is narrower and works more like a forceps than the larger-ended and more spoon-like bill of the royal spoonbill, which acts like a pair of tongs.

The yellow-billed spoonbill is carnivorous, catching small animals by sweeping its bill through shallow water and swallowing prey once it is detected.[5] When slow sweeping, the spoonbill walks with the bill at an angle at about 60 degrees to horizontal and with the bill tip open about 2 to 4 cm (1 to 1.5 in), sweeping an arc of around 120 degrees in front of the bird. The bird walks slowly, kicking up debris and small animals from the bottom of the water, which it then senses and catches with its bill. When an item is detected, the spoonbill switches to intensive sweeping of a small area.[6]

The yellow-billed spoonbill has a row of small blunt knobs known as papillae which line the margins of the upper and lower mandible of the "spoon".[6] These are sensory structures which help the bird sense vibration and hence seize its prey.[5]

One field study at Lake Cowal in New South Wales found the water depth selected for feeding to be less than 40 cm (15.5 in). As well as lakes and swamps, the birds feed in paddocks inundated after heavy rain. Foraging occurs both during the day and at night.[6]

Yellow-billed spoonbills also probe submerged plants directly for prey, and seize prey such as spiders above ground. They have been observed dragging their bills alongside themselves through shallow water while walking.[6]

Prey items recorded at Lake Cowal include freshwater crustaceans such as the common yabby (Cherax destructor); shrimp of the genus Macrobrachium and family Atyidae; insects, particularly aquatic bugs of the families Notonectidae and; fish such as mosquitofish) and flathead gudgeon (and occasionally freshwater snails and plant material such as medic burr