Justicia is a genus of flowering plants in the family Acanthaceae. It is the largest genus within the family, with over 900 accepted species. They are native to tropical to warm temperate regions of the Americas, India, and Africa. The genus serves as host to many butterfly species, such as Anartia fatima. Common names include water-willow and shrimp plant, the latter from the inflorescences, which resemble a shrimp in some species. The generic name honours Scottish horticulturist James Justice (1698–1763). They are closely related to Pachystachys.

Acanthaceae is a family of dicotyledonous flowering plants containing almost 250 genera and about 2500 species. Most are tropical herbs, shrubs, or twining vines; some are epiphytes. Only a few species are distributed in temperate regions. The four main centres of distribution are Indonesia and Malaysia, Africa, Brazil, and Central America. Representatives of the family can be found in nearly every habitat, including dense or open forests, scrublands, wet fields and valleys, sea coast and marine areas, swamps, and mangrove forests.