Brown sugar is a sucrose sugar product with a distinctive brown color due to the presence of molasses. It is by tradition an unrefined or partially refined soft sugar consisting of sugar crystals with some residual molasses content (natural brown sugar), but now often produced by the addition of molasses to refined white sugar (commercial brown sugar).

The Codex Alimentarius requires brown sugar to contain at least 88% of sucrose plus invert sugar. Commercial brown sugar contains from 3.5% molasses (light brown sugar) to 6.5% molasses (dark brown sugar) based on total volume. Based on total weight, regular commercial brown sugar contains up to 10% molasses. The product is naturally moist from the hygroscopic nature of the molasses and is often labeled "soft." The product may undergo processing to give a product that flows better for industrial handling. The addition of dyes or other chemicals may be permitted in some areas or for industrial products.

Particle size is variable but generally smaller than granulated white sugar. Products for industrial use (e.g., the industrial production of cakes) may be based on caster sugar, which has crystals of approximately 0.35mm.