The recent discovery of iron-based superconductors challenges the existing paradigm of high-temperature superconductivity. Owing to their unusual multi-orbital band structure, magnetism and electron correlation, theories propose a unique sign-reversed s-wave pairing state, with the order parameter changing sign between the electron and hole Fermi pockets. However, because of the complex Fermi surface topology and materials-related issues, the predicted sign reversal remains unconfirmed. Here we report a new phase-sensitive technique for probing unconventional pairing symmetry in the polycrystalline iron pnictides. Through the observation of both integer and half-integer flux-quantum transitions in composite niobium-iron pnictide loops, we provide the first phase-sensitive evidence of the sign change of the order parameter in NdFeAsO0.88F0.12, lending strong support for microscopic models predicting unconventional s-wave pairing symmetry. These findings have important implications on the mechanism of iron pnictide superconductivity, and lay the groundwork for future studies of new physics arising from the exotic order in the FeAs-based superconductors.