Heliotropism, a form of tropism, is the diurnal or seasonal motion of plant parts (flowers or leaves) in response to the direction of the Sun.

The habit of some plants to move in the direction of the Sun, a form of tropism, was already known by the Ancient Greeks. They named one of those plants after that property Heliotropium, meaning "sun turn". The Greeks assumed it to be a passive effect, presumably the loss of fluid on the illuminated side, that did not need further study. Aristotle's logic that plants are passive and immobile organisms prevailed. In the 19th century, however, botanists discovered that growth processes in the plant were involved, and conducted increasingly in-depth experiments. A. P. de Candolle called this phenomenon in any plant heliotropism (1832). It was renamed phototropism in 1892, because it is a response to light rather than to the sun, and because the phototropism of algae in lab studies at that time strongly depended on the brightness (positive phototropic for weak light, and negative phototropic for bright light, like sunlight). A botanist studying this subject in the lab, at the cellular and subcellular level, or using artificial light, is more likely to employ the more abstract word phototropism, a term which includes artificial light as well as natural sunlight. The French scientist Jean-Jacques d'Ortous de Mairan was one of the first to study heliotropism when he experimented with the Mimosa pudica plant. The phenomenon was studied by Charles Darwin and published in his penultimate 1880 book The Power of Movement in Plants, a work which included other stimuli to plant movement such as gravity, moisture and touch.