Certain ternary intermetallic compounds formed between rare‐earth and transition metals are of particular interest, since their high saturation magnetization and strong, uniaxial, magnetocrystalline anisotropy are of advantage for the practical applications as permanent magnets. The thermodynamics of the Fe‐Gd‐Mo system are assessed based on phase‐diagram information and the heat capacity data. The Fe‐Gd description was modified to account for the experimentally‐observed ternary phase equilibria involving binary phases. The assessment was done using a multi‐component thermodynamic software Thermo‐Calc. The assessment of the Fe‐Gd‐Mo ternary system shows reasonable agreement with the available experimental data. The calculated projection of the monovariant liquidus lines qualitatively reproduces the experimental one. The calculated isothermal sections are in general agreement with the experimental observations. However, the assessment can be improved with more data regarding the solidification behaviour and the magnetic properties of intermetallic phases.