The decay-time-dependent CP asymmetry in B0 s → J/ψ(→ μ+μ−)K+K− decays is measured using proton-proton collision data, corresponding to an integrated lu- minosity of 6 fb−1, collected with the LHCb detector at a center-of-mass energy of 13 TeV. Using a sample of approximately 349 000 B0 s signal decays with an invariant K+K− mass in the vicinity of the ϕ(1020) resonance, the CP -violating phase ϕs is measured, along with the difference in decay widths of the light and heavy mass eigen- states of the B0 s -B0 s system, ∆Γs, and the difference of the average B0 s and B0 meson decay widths, Γs − Γd. The values obtained are ϕs = −0.039 ± 0.022 ± 0.006 rad, ∆Γs = 0.0845 ± 0.0044 ± 0.0024 ps−1 and Γs − Γd = −0.0056 + 0.0013 − 0.0015 ± 0.0014 ps−1, where the first uncertainty is statistical and the second systematic. These are the most precise single measurements to date and are consistent with expectations based on the Standard Model and with the previous LHCb analyses of this decay. These results are combined with previous independent LHCb measurements. The phase ϕs is also measured independently for each polarization state of the K+K− system and shows no evidence for polarization dependence.