This paper reports the first ATLAS measurement of the B0(s) RIGHTWARDS ARROW μμ effective lifetime. The measurement is based on the data collected in 2015-2016, amounting to 26.3 fb-1 of 13TeV LHC proton-proton collisions. The proper decay-time distribution of 58+-13 background-subtracted signal candidates is fit with simulated signal templates parameterised as a function of the B0 s effective lifetime, with statistical uncertainties extracted through a Neyman construction. The resulting effective measurement of the B0 s RIGHTWARDS ARROW μμ lifetime is 0.99+0.42
-0.07 (stat.)+-0.17 (syst.) ps and it is found to be consistent with the Standard Model.