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We demonstrate a direct correlation between the domain structure of multiferroic BiFeO₃ thin films and exchange bias of Co_0.9Fe_0.1/BiFeO₃ heterostructures. Two distinct types of interactions − an enhancement of the coercive field (exchange enhancement) and an enhancement of the coercive field combined with large shifts of the hysteresis loop (exchange bias) − have been observed in these heterostructures, which depend directly on the type and crystallography of the nanoscale (∼2 nm) domain walls in the BiFeO₃ film. We show that the magnitude of the exchange bias interaction scales with the length of 109° ferroelectric domain walls in the BiFeO₃ thin films which have been probed via piezoresponse force microscopy and X-ray magnetic circular dichroism.