We present an experimental realization of resonance fluorescence in squeezed vacuum. We strongly couple microwave-frequency squeezed light to a superconducting artificial atom and detect the resulting fluorescence with high resolution enabled by a broadband traveling-wave parametric amplifier. We investigate the fluorescence spectra in the weak and strong driving regimes, observing up to 3.1 dB of reduction of the fluorescence linewidth below the ordinary vacuum level and a dramatic dependence of the Mollow triplet spectrum on the relative phase of the driving and squeezed vacuum fields. Our results are in excellent agreement with predictions for spectra produced by a two-level atom in squeezed vacuum [Phys. Rev. Lett. 58, 2539 (1987)], demonstrating that resonance fluorescence offers a resource-efficient means to characterize squeezing in cryogenic environments.United States. Army Research Office (W911NF-14-1-0078)United States. Office of Naval Research (N00014-13-1-0150)United States. Office of the Director of National IntelligenceUnited States. Intelligence Advanced Research Projects ActivityUnited States. Air Force (Contract No. FA8721-05-C-0002)United States. Air Force Office of Scientific Research. Multidisciplinary University Research Initiative (Grant No. FA9550-12-1-0488)