We report the detection of complex organic molecules in the young protostellar outflow Ll 157. We identify lines from HCOOCH₃, CH ₃CN, HCOOH, and C₂H₅OH at the position of the Bl shock in the blueshifted lobe, making it the first time that complex species have been detected toward a molecular outflow powered by a young low-mass protostar. The timescales associated with the warm outflow gas (<2 × 10³ yr) are too short for the complex molecules to have formed in the gas phase after the shock-induced sputtering of the grain mantles. It is more likely that the complex species formed in the surface of grains and were then ejected from the grain mantles by the shock. The formation of complex molecules in the grains of low-mass star forming regions must be relatively efficient, and our results show the importance of considering the impact of outflows when studying complex molecules around protostars. The relative abundance with respect to methanol of most of the detected complex molecules is similar to that of hot cores and molecular clouds in the Galactic center region, which suggests that the mantle composition of the dust in the L1157 dark cloud is similar to dust in those regions.