We present constraints on the existence of weakly interacting massive particles (WIMPs) from an 11 kg d target exposure of the DAMIC experiment at the SNOLAB underground laboratory. The observed energy spectrum and spatial distribution of ionization events with electron-equivalent energies >200 eVee in the DAMIC CCDs are consistent with backgrounds from natural radioactivity. An excess of ionization events is observed above the analysis threshold of 50 eVee. While the origin of this low-energy excess requires further investigation, our data exclude spin-independent WIMP-nucleon scattering cross sections σχ−n as low as 3×10−41 cm2 for WIMPs with masses mχ from 7 to 10 GeV c−2. These results are the strongest constraints from a silicon target on the existence of WIMPs with mχ<9 GeV c−2 and are directly relevant to any dark matter interpretation of the excess of nuclear-recoil events observed by the CDMS silicon experiment in 2013.