The angular distribution of the flavor-changing neutral current decay ({B}^{+}\to {K}^{+}{\mu }^{+}{\mu }^{-}) is studied in proton-proton collisions at a center-of-mass energy of 8 TeV. The analysis is based on data collected with the CMS detector at the LHC, corresponding to an integrated luminosity of (20.5\text{}\text{}{\mathrm{fb}}^{-1}). The forward-backward asymmetry ({A}{\mathrm{FB}}) of the dimuon system and the contribution ({F}{H}) from the pseudoscalar, scalar, and tensor amplitudes to the decay width are measured as a function of the dimuon mass squared. The measurements are consistent with the standard model expectations.