We report the measurements of the CP -violating parameters in B s 0  → D s ∓ K± decays observed in pp collisions, using a data set corresponding to an integrated luminosity of 3.0 fb−1 recorded with the LHCb detector. We measure C f = 0.73 ± 0.14 ± 0.05, A f ΔΓ  = 0.39 ± 0.28 ± 0.15, AΔΓf¯¯¯=0.31±0.28±0.15 , S f = −0.52 ± 0.20 ± 0.07, Sf¯¯¯=−0.49±0.20±0.07 , where the uncertainties are statistical and systematic, respectively. These parameters are used together with the world-average value of the B s 0 mixing phase, −2β s , to obtain a measurement of the CKM angle γ from B s 0  → D s ∓ K± decays, yielding γ = (128 − 22 + 17 )° modulo 180°, where the uncertainty contains both statistical and systematic contributions. This corresponds to 3.8 σ evidence for CP violation in the interference between decay and decay after mixing.