Cost-effective and efficient photocatalysis are highly desirable in chemical synthesis. Here we demonstrate that readily prepared suspensions of APbBr(3) (A = Cs or methylammonium (MA)) type perovskite colloids (ca. 2-100 nm) can selectively photocatalyze carbon-carbon bond formation reactions, i.e., alpha-alkylations. Specifically, we demonstrate alpha-alkylation of aldehydes with a turnover number (TON) of over 52,000 under visible light illumination. Hybrid organic/inorganic perovskites are revolutionizing photovoltaic research and are now impacting other research fields, but their exploration in organic synthesis is rare. Our low-cost, easy-to-process, highly efficient and bandedge-tunable perovskite photocatalyst is expected to bring new insights in chemical synthesis.