We report the attainment of the ferromagnetic state in an interstitially modified heavy rare-earth-iron intermetallic compound in an external magnetic field. The starting composition is RE₂Fe₁₇, which is the RE-Fe binary richest in iron. We concentrate on the Tm-Fe compound, which is the most sensitive to magnetic field. The maximum possible amount of hydrogen (5 at.H/f.u.) is inserted into a Tm₂Fe₁₇ single crystal. We demonstrate that in a magnetic field of 57 T Tm₂Fe₁₇H₅ reaches the ferromagnetic state with an enviably high polarization of 2.25 T.