Joseph Decaisne (7 March 1807 – January 1882) was a French botanist and agronomist.
Although he was born in Brussels, Belgium, he exercised his activity exclusively in Paris. He enters in 1824 as gardener to the Muséum national d'histoire naturelle (French Museum of natural history) and becomes, in 1832, chief of the sowing section.
He became the helper naturalist of the chair of rural botany of Adrien-Henri de Jussieu (1797-1853). It was there that he began to study plants brought back by various travelers like those of Victor Jacquemont (1801-1832) in Asia. He is also interested in algae, but it is mainly some applied research, notably on agronomy of the madder, the yam and the ramie.
In 1850, Decaisne followed Charles-François Brisseau de Mirbel (1776-1854) as the Chair of Culture of the Museum. In 1854 he participated in the creation of the Botanical Society of France.
The genera Decaisnea Hook.f. & Thomson (Lardizabalaceae), Decaisnea Brongn. syn. of Prescottia Lindl. and Decaisnea Lindl. syn. of Tropidia Lindl. (Orchidaceae), were named in his honour.
He died in Paris in January 1882.
He described the following genera of flowering plants: Capanea and Chrysothemis of the Gesneriaceae family; Sautiera (Acanthaceae); Lepinia, Rhazya (Apocynaceae); Vancouveria (with C.Morren) (Berberidaceae); Ostryopsis (Betulaceae); Dipterygium (Capparidaceae); Brassaiopsis, Cuphocarpus*, Dendropanax*, Didymopanax*, Fatsia*, Oreopanax*, Stilbocarpa*, (* with Planch.) (Araliaceae); Berneuxia (Diapensiaceae); Scyphogyne (Ericaceae); Akebia, Boquila (Lardizabalaceae); Galtonia (Liliaceae s. l. or Hyacinthaceae); Treculia Decne. ex Trecul (Moraceae; Camptotheca (Nyssaceae or Cornaceae); Ephippiandra (Monimiaceae); Pseudais (Thymelaeaceae); Allardia, Lecocarpus, Wollastonia DC. ex Decne. (Asteraceae); Gymnotheca (Saururaceae); Bougueria (Plantaginaceae); Docynia (Rosaceae); Seetzenia R.Br. ex Decne.(Zygophyllaceae); Deherainia (Theophrastaceae); Lopholepis (Poaceae); Asterostemma, Atherandra, Baeolepis Decne. ex Moq., Barjonia, Blepharodon, Calostigma, Camptocarpus, Decabelone, Decanema, Dictyanthus, Glossonema, Gongronema (Endl.) Decne. , Harpanema, Hemipogon, Hoodia Sweet ex Decne., Ibatia, Macropetalum Burch. ex Decne., Melinia, Mitostigma, Nautonia, Nephradenia, Orthosia, Pentopetia, Peplonia, Periglossum, Pherotrichis, Polystemma, Ptycanthera, Pycnoneurum, Pycnostelma Bunge ex Decne., Rhyssostelma, Riocreuxia, Tacazzea, Tassadia, Trichosandra ( Asclepiadaceae or Apocynaceae s. l.); Amorphophallus Blume ex Decne. (Araceae) and Leptopus (Euphorbiaceae s. l. or Phyllanthaceae).