Disturbance observer (DOB) is widely used in many practical applications due to its simple structure and high performance. However, the DOB cannot be directly applied to the robotic systems because of the nonlinearities/couplings in the inertia matrix (by means of coupling, we mean off-diagonal terms of the inertia matrix). This paper proposes a momentum-based DOB for general rigid joint robotic systems. By introducing the generalized momentum, it is possible to utilize full nonlinearities and couplings of the inertia matrix in the DOB design. Moreover, the momentum-based DOB design for the rigid joint robots can be easily extended to the flexible joint robot applications by applying it to the link-side dynamics and motor-side dynamics, respectively. As a result, we can estimate the external torque acting on the link-side and can compensate the disturbance occurring in the motor-side at the same time. Uniformly ultimated boundedness of the closed-loop dynamics can be shown through the Lyapunov-like approaches. The proposed scheme is verified using the numerical simulations.