This article presents a novel cascade state estimation framework for 3D Center of Mass (CoM) estimation of walking humanoid robots. The proposed framework, called SEROW (State Estimation RObot Walking) fuses effectively joint encoder, inertial, feet pressure and visual odometry measurements. Initially, we consider the humanoid’s Newton-Euler dynamics and rigorously derive the non-linear CoM estimator. The latter accurately estimates the 3D-CoM position, velocity and external forces acting on the CoM, while directly considering the presence of uneven terrain and the body’s angular momentum rate and thus effectively coupling the frontal with the lateral plane dynamics. Furthermore, we extend an established floating mass estimator to take into account the support foot pose, yielding in such a way the mandatory, for CoM estimation, affine transformations and forming a cascade state estimation scheme. Subsequently, we quantitatively and qualitatively assess the proposed scheme by comparing it to other estimation structures in terms of accuracy and robustness to disturbances, both in simulation and on an actual NAO robot walking outdoors over an inclined terrain. To facilitate further research endeavors, our implementation is offered as an open-source ROS/C++ package.