The characterization of tellurium selenide in its electronic ground state (X0+∑−) was possible by using LASER-ablation to isolate the diatomic in a pulsed molecular beam and by applying Fourier transform microwave spectroscopy to obtain the pure rotational spectrum. Employing a global multi-isotopologue analysis to transitions of 43 isotopologues of TeSe in seven vibrational states spectroscopic Dunham parameters Yℓ,m, Born–Oppenheimer breakdown coefficients δℓ,m, the equilibrium bond lengths re, as well as the vibration parameters ωe and ωexe were obtained for all analysed isotopologues. For low vibrational states, the Morse-potential function describes the TeSe-potential very well and provides an estimate of the maximum dissociation energy for this semi-metal compound. In addition, the isotopologue independent molecular constants Uℓ,m and the corresponding Born–Oppenheimer breakdown coefficients Δℓ,m were determined. Quite large coefficients and were necessary for Watson's reference isotopologue independent analysis. This is rationalized by the interaction between the two sublevels of the electronic 3Σ-state. Also the magnetic spin–rotation coupling constants that were achieved for some of the isotopologues reflect this electronic ground state. Field shift effects are found to be negligible.