This paper describes for the first time the summer and winter distributions of sea surface δ13CDIC in the Southern Indian Ocean (20°S–60°S). For this we used δ13CDIC measurements from 10 cruises conducted between 1998 and 2005. For summer and winter, the highest δ13CDIC values (>2‰) are observed in sub-Antarctic waters (40°S–50°S) and attributed mainly to biological activity, enhanced in the vicinity of Crozet and Kerguelen Archipelagoes. The lowest δ13CDIC values are found in subtropical waters (25°S–35°S), with a minimum (<1‰) in the Agulhas Current region and in the Mozambique channel. On the seasonal scale, δ13CDIC is higher during summer than during winter in all regions. The largest seasonal amplitude of variation (∼0.3‰), observed in the region 35°S–40°S, is attributed to biological activity during summer and to deep vertical mixing during winter. In subtropical oligotrophic waters, the mean seasonal amplitude of variation (∼0.15‰) is mainly explained by air–sea CO2 fluxes. On the interannual scale, we also identified a large negative anomaly of δ13CDIC in the subtropical waters during austral summer 2002, associated to an anomalous ocean CO2 sink due to cold conditions during this period.