A late-summer survey of herbicide-resistant (HR) weeds was conducted in Alberta in 2007, Manitoba in 2008, and Saskatchewan in 2009, totaling 1,000 randomly selected annually cropped fields. In addition, we screened 1,091 weed seed samples (each sample from one field) submitted by Prairie growers between 2007 and 2011. Of 677 fields where wild oat samples were collected, 298 (44%) had an HR biotype. Group 1 (acetyl CoA carboxylase inhibitor)-HR wild oat was confirmed in 275 fields (41%), up from 15% in previous baseline surveys (2001 to 2003). Group 2 (acetolactate synthase)-HR wild oat was found in 12% of fields (vs. 8% in 2001 to 2003). Group 8 (triallate, difenzoquat)-HR wild oat was identified in only 8% of fields (not tested in 2001 to 2003); the frequency of occurrence of group 1+2-HR wild oat was similar (8%, vs. 3% in 2001 to 2003). Group 1-HR green foxtail was found in 27% of 209 fields sampled for the weed (vs. 6% in 2001 to 2003). Group 2-HR spiny sowthistle was confirmed in all Alberta fields sampled (vs. 67% in 2001); common chickweed was found mainly in Alberta in 40% of fields (vs. 17% in 2001). Group 2-HR weed biotypes not previously detected in the baseline surveys included false cleavers mainly in Alberta (17% of fields) and Saskatchewan (21%), Powell amaranth in Manitoba (16% of fields), wild mustard (three populations in Saskatchewan and Manitoba), and wild buckwheat (one population in Alberta). No sampled weed populations across the Prairies were found to be resistant to herbicides from group 4 (synthetic auxins), group 9 (glyphosate), or group 10 (glufosinate). Based on the proportion of total field area at each site infested with HR weeds, it is estimated that 7.7 million ha (29% of annually cropped land) are infested with HR weeds (eight-fold increase from 2001 to 2003), in a total field area of 9.9 million ha (37%)—over a two-fold increase. Of 816 cases of HR wild oat identified from submitted samples, 69% were group 1-HR, 15% group 2-HR, and 16% group 1+2-HR. Additionally, there were 10 populations of group 1-HR green foxtail in Saskatchewan or Manitoba, and six populations of group 1-HR Persian darnel in southern Alberta and Saskatchewan. Various group 2-HR broadleaf weeds were identified, including 17 wild mustard populations mainly from Saskatchewan and 39 cleavers populations across the three Prairie provinces. Herbicide-use data from 2006 to 2010 indicated continued reliance on group 1 herbicides in cereal crops and group 2 herbicides in pulse crops.