植物激素（Plant hormone），又称植物荷尔蒙，是一些在植物体内合成，可以从产生部位输送至作用部位，微量浓度即可对植物体产生某种生理作用的活性有机物。亦可以人工合成结构作用相似的化学物质（植物生长调节剂），例如：1-萘乙酸（NAA）、2,4-二氯苯氧乙酸（2,4-D）。

目前得到公认的植物激素有五大类，包括：生长素类、赤霉素类（吉贝素）、细胞分裂素类、脱落酸（离层素）和乙烯。除此之外，还发现其它一些有激素生理活性的物质，如：甾类、多胺类、水杨酸类等。其中，生长素、赤霉素和细胞分裂素主要功能是促进生长的物质，脱落酸和乙烯则与植物器官的休眠、成熟和衰老有关。

Plant hormones (also known as phytohormones) are chemicals that regulate plant growth, which, in the UK, are termed 'plant growth substances'.

Plant hormones are signal molecules produced within the plant, and occur in extremely low concentrations. Hormones regulate cellular processes in targeted cells locally and, moved to other locations, in other functional parts of the plant. Hormones also determine the formation of flowers, stems, leaves, the shedding of leaves, and the development and ripening of fruit. Plants, unlike animals, lack glands that produce and secrete hormones. Instead, each cell is capable of producing hormones. Plant hormones shape the plant, affecting seed growth, time of flowering, the sex of flowers, senescence of leaves, and fruits. They affect which tissues grow upward and which grow downward, leaf formation and stem growth, fruit development and ripening, plant longevity, and even plant death. Hormones are vital to plant growth, and, lacking them, plants would be mostly a mass of undifferentiated cells. So they are also known as growth factors or growth hormones. The term 'Phytohormone' was coined by Thimann in 1948.