We consider the problem of power allocation for capacity maximization in a decode-and-forward (DF) based multi-Hop MIMO-OFDM systems. We derive the necessary conditions for the optimal solution and propose an optimal power allocation algorithm for a 3-hop system, which can be easily extended to arbitrary number of relays. A suboptimal power allocation algorithm for a 3-hop system is also proposed. The suboptimal solution assumes equal power allocation over frequency domain and uses closed-form expressions for optimal power distribution over all hops for each subcarrier. Simulation results verify that, given the same transmit power, edge users are better served by a multi-hop relay scheme than by direct-link based scheme. Moreover, the proposed suboptimal power allocation yields an excellent sum rate performance which is close to the optimal performance.