The B s0→J/ψK S0 branching fraction is measured in a data sample corresponding to 0.41fb -1 of integrated luminosity collected with the LHCb detector at the LHC. This channel is sensitive to the penguin contributions affecting the sin2β measurement from B 0→J/ψK S0. The time-integrated branching fraction is measured to be B(B S0→J/ψK S0)=(1.83±0.28)×10 -5. This is the most precise measurement to date. © 2012 CERN.