The mechanical characteristics, phase composition, residual macroscopic stresses, and surface topography of the Co–Cr–Mo–W alloy manufactured by 3$D$ printing using laser bed-powder fusion and modified by ultrasonic impact treatment (UIT) in an inert environment are studied. The interrelation between the mechanical properties of the modified surface layers and their structural and phase state after UIT of different durations (20–60 s) is established. The maximum hardening of the surface layer of the Co–Cr–Mo–W alloy (by 1.5 times) is achieved after UIT for 30 s and is caused by the formation of compressive stresses of the first kind (-510 MPa, which is $\cong$ 5.5 times higher as compared to the initial state), a decrease in the crystallite sizes of the $\varepsilon$- (80 nm) and $\gamma$- ($\cong$ 140 nm) phases, as well as an increase in the content of the $\varepsilon$-phase from 5% to 95% due to the martensitic transformation. The roughness of the modified surface after the optimal UIT regime (30 s) has parameters that practically do not differ from the initial polished state ($R_{a}$ = 2.66 µm and $R_{z}$ = 0.52 µm). The increase in microhardness and wear resistance of the surface layers proves the possibility of using UIT as an effective method for local (surface) post-treatment of medical products made of 3$D$-printed Co–Cr–Mo–W alloy.