Campanula rotundifolia is a perennial species of flowering plant, a slender, prostrate to erect herb, spreading by seed and rhizomes. The basal leaves are long-stalked, rounded to heart-shaped, usually slightly toothed, with prominent hydathodes, and often wither early. Leaves on the flowering stems are long and narrow and the upper ones are unstemmed. The inflorescence is a panicle or raceme, with 1 – many flowers borne on very slender pedicels. The flowers usually have five (occasionally 4, 6 or 7) pale to mid violet-blue petals fused together into a bell shape, about 12–30 mm (0.5–1.2 in) long and five long, pointed green sepals behind them. Plants with pale pink or white flowers may also occur. The petal lobes are triangular and curve outwards. The seeds are produced in a capsule about 3–4 mm (0.1–0.2 in) diameter and are released by pores at the base of the capsule. Seedlings are minute, but established plants can compete with tall grass. As with many other Campanulas, all parts of the plant exude white latex when injured or broken.

The flowering period is long, and varies by location. The flowers are pollinated by bees, but can self-pollinate.