Perennial wall-rocket is a glabrous herb with an erect or sprawling habit, that grows up to 1.3 m tall, with a solid, almost woody terete stem and spreading branches. The deeply pinnate leaves are up to 12 cm long and often rather fleshy, with a peppery taste and a musty smell.

In the British Isles, it flowers from May to September (or through October in a warm year). In Spain, it can be seen flowering between April and December. The inflorescence is a branched raceme up to 30 cm long with up to 30 flowers, each of which has 4 free bright yellow petals up to 15 mm long, and 4 free yellow/brown sepals up to 7.5 mm long. Each flower has 6 stamens and a single style.

The fruit is a straight, flat silique (pod) up to five centimeters long. The pedicels are shorter than the fruits and ascend at an acute angle to the stem. Above the receptacle is a short (2 mm) stalk (or stipe) below the pod (a useful feature for separating this species from annual wall-rocket). The seeds are arranged in 2 rows, or staggered in a zigzag pattern, towards the centre of the pod, which in turn consists of 2 valves, so a cross-section of the fruit shows 4 seeds in total. This is a distinguishing feature of the genus Diplotaxis, although it is not always easy to see, as plants are not self-pollinating, so ripe seeds do not always develop.