向光性（英语：phototropism）是向性的一种，指生物的生长由光源的方向而影响的性质，常见于植物之中。朝向有光的一面生长称为正向光性，反之称为负向光性。大部分植物的茎部都是正向光性的，这可帮助叶的光合作用。而根部通常是负向光性的，不过向地性对根部生长方向更为重要。

在部分植物中，茎部的正向光性是由一种由茎顶分泌的生长素（英语：auxin）造成的。受光的一面生长素合成受阻，背光的一面因此会含有更多的生长素并向下运输。生长素一方面能够刺激细胞膜上的质子泵，质子由胞浆流入胞壁，造成胞壁酸化。氢键和细胞壁多糖之间的键裂解，细胞壁松弛，细胞水势下降。细胞外水分流入造成细胞的伸长生长。另一方面，生长素会在基因水平上调节蛋白质合成，如周期蛋白，促进植物生长。

Phototropism is the growth of an organism which responds to a light stimulus. It is most often observed in plants, but can also occur in other organisms such as fungi. The cells on the plant that are farthest from the light have a chemical called auxin that reacts when phototropism occurs. This causes the plant to have elongated cells on the farthest side from the light. Phototropism is one of the many plant tropisms or movements which respond to external stimuli. Growth towards a light source is called positive phototropism, while growth away from light is called negative phototropism. Most plant shoots exhibit positive phototropism, and rearrange their chloroplasts in the leaves to maximize photosynthetic energy and promote growth. Roots usually exhibit negative phototropism, although gravitropism may play a larger role in root behavior and growth. Some vine shoot tips exhibit negative phototropism, which allows them to grow towards dark, solid objects and climb them. The combination of phototropism and gravitropism allow plants to grow in the correct direction.