Budgerigars are small, streamlined parrots that average 18-20 cm in length. They are unlikely to be mistaken for any other parrot because of their small size, pointed wings and tails and distinct plumage pattern. Most wild budgerigars have a yellow forehead (juveniles have a barred forehead), a yellow and black striped head with purple and black markings on the cheeks, a pointed bill whose tip of the upper mandible extends over the lower mandible, and a yellow throat. Their lesser and median wing coverts are centred black and outlined in yellow. Both their greater coverts and flight feathers are centred black and outlined with green and yellow, but their flight feathers also have a central yellow bar. Their uppertail coverts are bright green and extend to a blue-green tail. Caged species differ greatly in their plumage colour and patterns.

There is slight sexual dimorphism. In breeding females, the cere (the skin at the base of the bill, covering the nostrils) is light brown or beige. Otherwise the cere is blue.