A search for the dimuon decay of the Standard Model Higgs boson was performed using data corresponding to an integrated luminosity of $79.8$ fb$^{-1}$ collected so far in Run 2 with the ATLAS detector in $pp$ collisions at $\sqrt{s}=13$ TeV at the Large Hadron Collider. No significant excess is observed above the expected background. The observed upper limit on the cross section times branching ratio is $2.1$ times the Standard Model prediction at $95$\% confidence level for a Higgs boson mass of $125$ GeV, while the expected limit in the absence of a $H \to \mu\mu$ signal was $2.0$. The measured signal strength, defined as the ratio of the observed signal yield to the one expected in the Standard Model, is $\mu = 0.1 ^{+1.0}_{-1.1}$.

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