Two new 3,4-epoxy group-containing abietane diterpenoids (<b>1</b> and <b>2</b>), together with five known diterpenoids (<b>3</b>-<b>7</b>), were isolated from <i>Ajuga decumbens</i>. Their structures were elucidated by spectroscopic data analysis, NMR calculations, and X-ray diffraction experiments. The structures of two known abietane diterpenoids were revised based on NMR calculations and X-ray diffraction data. Notably, compound <b>4</b> specifically inhibited RSL3-induced ferroptosis with an EC<sub>50</sub> of 56 nM by antioxidation. Moreover, <b>4</b> significantly decreased RSL3-induced lipid and cytosolic ROS accumulation and ferroptosis marker gene <i>PTGS2</i> mRNA expression. This work reports the most potent natural inhibitor against ferroptosis found so far.