Brassicasterol (24-methyl cholest-5,22-dien-3β-ol) is a 28-carbon sterol synthesised by several unicellular algae (phytoplankton) and some terrestrial plants, like rape.

In anaerobic sediments and soils, brassicasterol is stable for many hundreds of years, enabling it to be used as an indicator of past algal production (see below).

Since the molecule has a hydroxyl (-OH) group, it is frequently bound to other lipids including glycerols; most analytical methods, therefore, utilise a strong alkali (KOH or NaOH) to saponify the ester linkages.

Instrumental analysis is frequently conducted on gas chromatograph (GC) with either a flame ionisation detector (FID) or mass spectrometer (MS).

The location of brassicasterol in this figure (shown in red) indicates that the distribution of this compound is similar to that of the short-chain fatty acids and alcohols, which are known to be of marine origin.