Syzygium jambos is a large shrub or small-to-medium-sized tree, typivally three to 15 metres high, with a tendency to low branching. Its leaves and twigs are glabrous and the bark, though dark brown, is fairly smooth too, with little relief or texture. The leaves are lanceolate, 2cm to 4 cm broad, 10 cm to 20 cm long, pointed, base cuneate with hardly any petiole, lively red when growing, but dark, glossy green on attaining full size.
The flowers are in small terminal clusters, white or greenish white, the long, numerous stamens giving them a diameter of 5 - 8 cm. In temperate regions the tree is summer-flowering.
The edible fruit of Syzygium jambos is shaped like some kinds of guava, to which the plant is fairly closely related. In fact the fruit is so like the guava in appearance that people unfamiliar with it may mistake it for a guava on sight. However, the fragrance, flavour and texture are different, and instead of containing dozens of small, hard seeds set in a jelly-like tissue, as a guava does, the fruit of Syzygium jambos usually contains one or two large, unarmoured seeds about a cm in diameter, lying loose in a slightly fluffy cavity when ripe. Shaking a fruit to feel whether the seeds rattle, gives some indication whether it is ripe. The skin is thin and waxy. The flowers are described by some as fragrant, though this appears to be a variable attribute. The ripe fruit however, has a strong, pleasant floral bouquet, hence such common names as "Rose apple" and "pomarrosa".