This paper reports the first ATLAS measurement of the $ {B}_s^0 $→ μμ effective lifetime. The measurement is based on the data collected in 2015–2016, amounting to 26.3 fb$^{−1}$ of 13 TeV 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 $ {B}_s^0 $ effective lifetime, with statistical uncertainties extracted through a Neyman construction. The resulting effective measurement of the $ {B}_s^0 $→ μμ lifetime is $ {0.99}_{-0.07}^{+0.42} $ (stat.) ± 0.17 (syst.) ps and it is found to be consistent with the Standard Model.