Bitter orange; Seville orange; sour orange; bigarade orange; or marmalade orange refers to a citrus tree (Citrus aurantium) and its fruit. It is hybrid between Citrus maxima (pomelo) and Citrus reticulata (mandarin). Many varieties of bitter orange are used for their essential oil; which is used in perfume; as a flavoring and as a solvent. The Seville orange variety is used in the production of marmalade.

Bitter orange is also employed in herbal medicine as a stimulant and appetite suppressant; due to its active ingredient; synephrine. Bitter orange supplements have been linked to a number of serious side effects and deaths; and consumer groups advocate avoiding medicinal use of the fruit Pictures From History