A detailed investigation of the structure and magnetic properties of ${\mathrm{Tm}}_{\mathrm{2}}{\mathrm{Fe}}_{\text{17−x}}{\mathrm{Ga}}_x$ $\text{(x=0,}$ 1, 2, 3, 4, 5, 6, 7, and 8) compounds has been performed by means of x-ray-diffraction, neutron-diffraction, magnetization, and ac-susceptibility measurements. Crystal-structure studies have shown that the prepared samples are single phase with the hexagonal ${\mathrm{Th}}_{\mathrm{2}}{\mathrm{Ni}}_{\mathrm{17}}$ for $\text{x⩽3}$ and the rhombohedral ${\mathrm{Th}}_{\mathrm{2}}{\mathrm{Zn}}_{\mathrm{17}}$ structure for $\text{x⩾5.}$ In ${\mathrm{Tm}}_{\mathrm{2}}{\mathrm{Fe}}_{\mathrm{13}}{\mathrm{Ga}}_{\mathrm{4}}$ the ${\mathrm{Th}}_{\mathrm{2}}{\mathrm{Zn}}_{\mathrm{17}}$ structure coexists with the ${\mathrm{Th}}_{\mathrm{2}}{\mathrm{Ni}}_{\mathrm{17}}$ structure. Substitution of Ga for Fe in ${\mathrm{Tm}}_{\mathrm{2}}{\mathrm{Fe}}_{\mathrm{17}}$ leads to an increase of the unit-cell volume, which is linear with the Ga concentration. In ${\mathrm{Tm}}_{\mathrm{2}}{\mathrm{Fe}}_{\text{17−x}}{\mathrm{Ga}}_x,$ the saturation magnetization at 1.5 K decreases linearly with increasing Ga content with a rate of ${\text{2.3\hspace{0.17em}μ}}_B$ per substituted Ga atom. The Curie temperature is found first to increase with increasing Ga content, going through a maximum value of 485 K at about $\text{x=3,}$ then to decrease. Between $\text{x=6}$ and 7, a minimum value of $T_C$ is reached and for higher $x$ values $T_C$ increases again. X-ray-diffraction measurements on magnetically aligned ${\mathrm{Tm}}_{\mathrm{2}}{\mathrm{Fe}}_{\text{17−x}}{\mathrm{Ga}}_x$ powders show that the compounds with $\text{x⩽6}$ have an easy-plane type of magnetic anisotropy, whereas the compounds with $\text{x⩾7}$ exhibit easy $c$-axis anisotropy at room temperature. All ${\mathrm{Tm}}_{\mathrm{2}}{\mathrm{Fe}}_{\text{17−x}}{\mathrm{Ga}}_x$ compounds exhibit a spin-reorientation transition at low temperature, except for the sample with $\text{x=6,}$ which shows an easy-magnetization direction perpendicular to the $c$ axis in the temperature range from 5 to 300 K. For $\text{x⩽5,}$ the spin-reorientation temperature is found first to increase with $x$ and then to decrease, having a maximum value of 211 K at about $\text{x=3.}$ In the samples with $\text{x⩾7,}$ an easy-plane anisotropy was observed at low temperature, but an easy-axis preference of the magnetization at room temperature was observed. The results obtained for ${\mathrm{Tm}}_{\mathrm{2}}{\mathrm{Fe}}_{\text{17−x}}{\mathrm{Ga}}_x$ indicate that the mutually competing Tm- and Fe-sublattice anisotropies both change their sign with increasing Ga concentration.