Results on B → μ+μ− decays with the CMS experiment are reported, using 61 fb−1 of data recorded during LHC Run 1 and 2016. With an improved muon identification algorithm and refined unbinned maximum likelihood fitting methods, the decay Bs0 → μ+μ− is observed with a significance of 5.6 standard deviations. Its branching fraction is measured to be B(Bs0 → μ+μ−) = [2.9±0.7 (exp)±0.2 (frag)]×10−9, where the first error is the combined statistical and systematic uncertainty and the second error quantifies the uncertainty of the Bs0 and B+ fragmentation probability ratio. The Bs0 → μ+μ− effective lifetime is τ = 1.70 +0.61-0.44 ps. No evidence for the decay B0 → μ+μ− is found and an upper limit of B(B0 → μ+μ−) < 3.6 × 10−10 (at 95% confidence level) is determined. All results are consistent with the standard model of particle physics.