In the Standard Model, the weak mixing angle sets the relative coupling strength of the weak hypercharge and weak isospin gauge symmetries. It is precisely predicted including higher order corrections; direct measurements are sensitive to possible effects of physics beyond the Standard Model. It is also a key parameter in global studies of the electroweak sector. New measurements are essential given the long-standing 3sigma discrepancy between the two best measurements to date at LEP and SLD, and in the light of the anomalously large W boson mass measured by CDF. A new result based on data from the LHCb experiment is presented, making use of the proton-proton collision dataset recorded during 2016-2018 at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.3 fb$^{-1}$. The forward-backward asymmetry in the $pp \to Z/\gamma^{*} \to \mu^+\mu^-$ process is measured, and theoretical predictions are used to translate this to a determination of the effective leptonic weak mixing angle. The result is compared with previous measurements of the weak mixing angle and with the predicted value from the global electroweak fit.