In this work, the 5P_3/2 → 6P_3/2 electric-dipole-forbidden transition in atomic rubidium at room temperature is studied in the presence of a magnetic field in the weak and strong field regimes. The experiment is performed in a rubidium cell with two external cavity diode lasers (ECDLs) in a counterpropagating configuration. A 780 nm laser at the D2 electric-dipole transition prepares atoms in the 5P_3/2 state, and a 911 nm laser produces the 5P_3/2 → 6P_3/2 electric-quadrupole transition. Both beams are linearly polarized in the direction of the magnetic field. Detection of atoms in the 6P_3/2 state is monitored by the 420 nm fluorescence decay (6P_3/2 → 5S_1/2) via current-modulated phase detection. The experimental geometry determines the electric-dipole (ΔM_F = 0) and electric-quadrupole (ΔM_F = ±1) hyperfine selection rules. Breit–Rabi diagrams of all involved states and their differences using these selection rules are presented. This allows for the identification of the resonant frequencies of the spectral lines as functions of the magnetic field in both the weak and the strong limits. Theoretical predictions agreed with the experimental data. Future work includes the calculation of line intensities via transition probabilities and a population equations model.