Adding Au nanoparticles to YBa2Cu3O7−δ thin films leads to an increase of the superconducting transition temperature Tc and the critical current density jc. While the higher jc can be understood in terms of a stronger pinning of the flux vortices at the Au nanoparticles, the enhanced Tc is still puzzling. In the present study, we determined the microscopic magnetic penetration profiles and the corresponding London penetration depths λL in the Meissner state of optimally doped YBa2Cu3O7−δ thin films with and without Au nanoparticles by low-energy muon spin rotation. By Rutherford backscattering spectrometry, we show that the Au nanoparitcles are distributed over the whole thickness of the thin-film samples. The superfluid density ns∝1/λ2L was found to increase in the films containing Au nanoparticles. We attribute this increase of ns to a reduction of the defect density possibly due to defect condensation at the Au nanoparticles.