•A momentum-based observer for quadruped robots is proposed.•The observer estimates forces acting on both stance and swing legs.•Whole-body controller with a quadratic problem is used.•Disturbances on swing legs are compensated using operational space.•The observer is compared with two from the state-of-the-art. This paper presents an estimator of external disturbances for legged robots, based on the system’s momentum. The estimator, along with a suitable motion planner for the trajectory of the robot’s center of mass and an optimization problem based on the modulation of ground reaction forces, devises a whole-body controller for the robot. The designed solution is tested on a quadruped robot within a dynamic simulation environment. The quadruped is stressed by external disturbances acting on stance and swing legs indifferently. The proposed approach is also evaluated through a comparison with two state-of-the-art solutions.