Alterbrassinoids A-D (1-4), the first examples of fusicoccane-derived diterpenoid dimers furnished by forming an undescribed C-12-C-18' linkage, were isolated from modified cultures of Alternaria brassicicola. Compounds 1 and 2 represent unprecedented heterodimers, whereas 3 and 4 represent unprecedented homodimers, and 4 also features an undescribed anhydride motif. Their structures were assigned via spectroscopic methods, electronic circular dichroism calculations, and single-crystal X-ray diffraction. Putative biosynthetic pathways and a bioactivity evaluation for 1-4 were discussed.