We explore biocarbon as a low-cost, abundant, and environmentally friendly replacement for Pt in dye solar cells. We introduce a novel biochar based on brewery residues with good performance and stability potential as a counter electrode in complete dye solar cells, and present the first long-term stability test results of a biocarbon in complete dye solar cells. The hydrothermally carbonized and KOH-activated brewer's spent grain (BSG) offers an extremely high surface area for catalytic reactions (2190 m2/g). Counter electrodes based on this material provide a promising initial performance (efficiency of 3.6 ± 0.2% for biocarbon solar cells compared to 5.3 ± 0.2 for reference cells with Pt catalyst) with current production and the total resistance of solar cells very close to that of Pt based solar cells. In an extended accelerated aging test, the best biocarbon dye solar cell maintained over 86% of its initial efficiency for 3000 h. Moreover, the biocarbon reduced the degradation via loss of electrolyte charge carriers during aging. Based on these results, the activated BSG biocarbon provides a promising alternative for Pt catalysts.