This species shows an extreme sexual dimorphism both in size and coloration, also in comparison with othe crab spiders. The adult males reach a body length of only 2–4 millimetres (0.079–0.157 in), while females are 7–10 millimetres (0.28–0.39 in) long. This species is characterized by the prominent rear corners of the opisthosoma. In males the basic colour of the prosoma varies from yellow brown to dark brown, the opisthosoma may be yellow and green or brown. Also females are very variable in colour, their basic colour can be white, yellow or pink. As a matter of fact these crab-spiders hide themselves adapting the colour of their body to the colour of the flowers on which they are waiting for preys, a behaviour that conceal them from predators and from the pollinating preys.
These spiders prefer warm temperatures, forest-free areas, dry and sandy habitats with high solar radiation and dry grasslands.
Like other species of the family Thomisidae these crab spiders do not make a web, but actively pursue their preys. They usually wait for preys positioned for hunting on flowers. Their victims are disguised by assuming the same colour as the flower, fooling both insect and bird predators. The prey consists of flower-visiting insects of all species, such as hover flies, bees, wasps, butterflies or beetles, which are often several times larger than the spider. They take their preys with two powerful and highly enlarged front leg pairs and usually kill them by biting on the back of their neck. Emerging spiderlings of Thomisus onustus may feed on pollen or nectar when insect preys are lacking.