The rainbow lorikeet is a medium-sized parrot, with the length ranging from 25 to 30 cm (10 to 12 in), including the tail. The weight varies from 75 to 157 g (2.6–5.5 oz). The plumage of the nominate race, as with all subspecies, is very bright. The head is deep blue with a greenish-yellow nuchal collar, and the rest of the upper parts (wings, back and tail) are deep green. The chest is red with blue-black barring. The belly is deep green, and the thighs and rump are yellow with deep green barring. In flight a yellow wing-bar contrasts clearly with the red underwing coverts.

There is little to visually distinguish between the sexes; however, to a keen observer of their colouring and behaviour, their dimorphism is readily apparent.

Juveniles have a black beak, which gradually brightens to orange in the adults.

The markings of Trichoglossus moluccanus resemble those of the Coconut Lorikeet ), but with a blue belly and a more orange breast with little or no blue-black barring.[10]

Rainbow lorikeets often travel together in pairs and occasionally respond to calls to fly as a flock, then disperse again into pairs. Rainbow lorikeet pairs defend their feeding and nesting areas aggressively against other rainbow lorikeets and other bird species. They chase off not only smaller birds such as the noisy miner and the little wattlebird, but also larger birds such as the Australian magpie.

Rainbow lorikeets feed mainly on fruit, pollen and nectar, and possess a tongue adapted especially for their particular diet. The end of the tongue is equipped with a papillae appendage adapted to gathering pollen and nectar from flowers.[13] Nectar from eucalyptus is important in Australia, other important nectar sources are Pittosporum, Grevillea, (African tulip-tree), and (sago palm).[10] In Melanesia coconuts are very important food sources, and rainbow lorikeets are important pollinators of these.[14] They also consume the fruits of Ficus, Trema, Mutingia, as well as papaya and mangoes already opened by fruit bats. They also eat crops such as apples, and will raid maize and sorghum.[10] They are also frequent visitors at bird feeders placed in gardens, which supply store-bought nectar, sunflower seeds, and fruits such as apples, grapes and pears.

In many places, including campsites and suburban gardens, wild lorikeets are so used to humans that they can be hand-fed. The Currumbin Wildlife Sanctuary in Queensland, Australia, is noted for its thousands of lorikeets. Around 8am and 4pm each day the birds gather in a huge, noisy flock in the park's main area. Visitors are encouraged to feed them a specially prepared nectar, and the birds will happily settle on people's arms and heads to consume it. Wild rainbow lorikeets can also be hand-fed by visitors at Lone Pine Koala Sanctuary in Brisbane, Queensland, Australia.

Semi-tame lorikeets are common daily visitors in many Sydney backyards, though many people, ignorant of their dietary requirements, feed them bread or bread coated with honey. This is an inadequate source of the nutrients, vitamins and minerals that the rainbow lorikeet requires and can lead to health and feather formation issues in young lorikeets .Packet mixes with a nutritional mix suitable for feeding lorikeets are generally available from vets and pet stores.[1