In this paper, we investigate distributed differential space-time block coding (STBC) for cooperative communications over frequency-selective fading channels. We carefully exploit the unitary structure of the orthogonal STBC to design a low complexity differential STBC receiver for multi-carrier broadband cooperative networks. We consider amplify-and-forward relaying and assume a single-relay scenario. Under the assumption of perfect power control for the relay terminal and high signal-to-noise ratio for the underlying links, our performance analysis demonstrates that the considered scheme is able to exploit fully the spatial diversity. We further present Monte Carlo simulation results to confirm our analytical observations.