This study considers the problem of robust interconnection and damping assignment passivity‐based control approach (IDA‐PBC) for underactuated mechanical systems from two points of view. First, the robustness of IDA‐PBC in the presence of non‐vanishing matched and unmatched uncertainties is analysed and sufficient conditions are derived to ensure ultimate boundedness of system. Second, the robust control design is provided by adding a new control input to the former controller designed by IDA‐PBC, such that asympt…