<jats:title>Abstract</jats:title><jats:p>Chemical investigation of the roots of <jats:italic>Linum usitatissimum</jats:italic> led to the isolation and characterization of a new diterpenoid, named usitatissimin A (<jats:bold>1</jats:bold>), together with four known compounds. The structure of the new compound was established by spectroscopic methods, including X‐ray crystallographic diffraction analysis that confirmed the relative configuration of <jats:bold>1</jats:bold>. This is the first example of a diterpenoid with a novel 6/6/6/3‐membered ring system of 14,15‐cyclopimarane. In addition, usitatissimin A showed pronounced hepatoprotective activity against <jats:sc>D</jats:sc>‐galactosamine‐induced toxicity in WB‐F344 rat hepatic epithelial stem‐like cells.