Recently, correlations have been reported between fluctuations in nuclear decay rates and Earth-Sun distance, which suggest that nuclear decay rates may be affected by solar activity. In this paper, we report the detection of a significant decrease in the decay of 54Mn during the solar flare of 2006 December 13, whose X-rays were first recorded at 02:37 UT (21:37 EST on 2006 December 12). Our detector was a 1 μCi sample of 54Mn, whose decay rate exhibited a dip coincident in time with spikes in both the X-ray and subsequent charged particle fluxes recorded by the Geostationary Operational Environmental Satellites (GOES). A secondary peak in the X-ray and proton fluxes on December 17 at 12:40 EST was also accompanied by a coincident dip in the 54Mn decay rate. These observations support the claim by Jenkins et al. that nuclear decay rates may vary with Earth-Sun distance.