An accurate description of noncovalent interaction energies is one of the most challenging tasks in computational chemistry. To date, nonempirical CCSD(T)/CBS has been used as a benchmark reference. Its practical use is, however, limited due to the rapid growth of its computational cost with the system complexity. Here we show, that the fixed-node diffusion Monte Carlo (FN-DMC) method with more favourable scaling is capable of reaching the CCSD(T)/CBS within subchemical accuracy (< 0.1 kcal/mol) on a testing set of six small noncovalent complexes including the water dimer. In benzene/water, benzene/methane, and the T-shape benzene dimer, FN-DMC provides interaction energies that agree within 0.25 kcal/mol with the best available CCSD(T)/CBS estimates. The demonstrated predictive power of FN-DMC therefore provides new opportunities for studies of the vast and important class of medium/large noncovalent complexes.