Sweet violet is a perennial winter—green terrestrial herbaceous plant without a leafy stem up to 15 cm high, with a thick creeping rhizome giving numerous rosettes of basal leaves and aboveground shoots taking root in nodes (stolons). Stolons are long, thin, 1.5—2 mm in diameter, usually well pronounced.
The leaves are simple, all collected in a basal rosette, together with a petiole no more than 15 cm long. Leaf blade with the greatest width near the middle, almost rounded, less often kidney-shaped, deep-heart-shaped at the base and slightly short-pointed at the tip, along the edges of the city-sawed.
The whole plant, including petioles, peduncles and usually boxes, is densely pubescent with large hairs.
The flowers are solitary, on pedicels, develop in the axils of the basal leaves. The perianth is double, the division of the perianth into a corolla and a calyx is quite distinct, the petals are all free. There are five petals in zygomorphic flowers, dark purple, less often white; the lower petal is slightly wider than the others, with a spur, the side petals are directed downward. Flowers with a pleasant strong aroma.
Blooms in April—early May and for the second time in late summer, bears fruit in June.
The fruit is cenocarp — spherical triangular, pubescent with short hairs, a single-nesting greenish box 3-5 mm in diameter, with an elongated seed arrangement, surrounded by a preserved calyx and opening with three flaps at the place of fusion of the carpels.