We present a measurement of the effective weak mixing angle parameter sin 2 θ ℓ eff in p ¯ p → Z / γ ∗ → μ + μ − events at a center-of-mass energy of 1.96 TeV, collected by the D0 detector at the Fermilab Tevatron Collider and corresponding to 8.6 fb − 1 of integrated luminosity. The measured value of sin 2 θ ℓ eff [ μ μ ] = 0.23016 ± 0.00064 is further combined with the result from the D0 measurement in p ¯ p → Z / γ ∗ → e + e − events, resulting in sin 2 θ ℓ eff [ comb ] = 0.23095 ± 0.00040 . This combined result is the most precise measurement from a single experiment at a hadron collider and is the most precise determination using the coupling of the Z / γ ∗ to light quarks.