This paper presents the development of a nonlinear controller for the reaction wheel pendulum (RWP) via an interconnection and damping assignment passivity-based control (IDA-PBC) approach. The IDA-PBC approach works with the port-Hamiltonian open-loop dynamics of the RWP to propose a nonlinear controller that preserves the Hamiltonian structure in closed-loop by guaranteeing stability properties in the sense of Lyapunov. Numerical results confirm the theoretical development presented throughout simulations in Simulink package from MATLAB. Comparison with a Lyapunov-based approach is also provided.