The K + K + scattering length is calculated in fully-dynamical lattice QCD with domain-wall valence quarks on the MILC asqtad-improved gauge configurations with rooted-staggered sea quarks. Three-flavor mixed-action chiral perturbation theory at next-to-leading order, which includes the leading effects of the finite lattice spacing, is used to extrapolate the results of the lattice calculation to the physical value of m K + / f K + . We find m K + a K + K + = − 0.352 ± 0.016 , where the statistical and systematic errors have been combined in quadrature.