A three-component hydrocarboxylation of an olefin with CO and H could be regarded as a dream reaction, since it would provide a straightforward approach for the synthesis of aliphatic carboxylic acids in perfect atom economy. However, this transformation has not been realized in a direct manner under mild conditions, because boosting the carboxylation with thermodynamically stable CO while suppressing the rapid hydrogenation of olefin remains a challenging task. Here, we report a rhodium-catalysed reductive hydrocarboxylation of styrene derivatives with CO and H under mild conditions, in which H served as the terminal reductant. In this approach, the carboxylation process was largely accelerated by visible light irradiation, which was proved both experimentally and by computational studies. Hydrocarboxylation of various kinds of styrene derivatives was achieved in good yields without additional base under ambient pressure of CO/H at room temperature. Mechanistic investigations revealed that use of a cationic rhodium complex was critical to achieve high hydrocarboxylation selectivity.