The issue of cross- or multiple resistance to acetolactate synthase (ALS) inhibitors and the auxinic herbicide 2,4-D was investigated in Papaver rhoeas L., a common and troublesome weed in winter cereals, in a broad-scale study across four European countries. A combination of herbicide sensitivity bioassays and molecular assays targeting mutations involved in resistance was conducted on 27 populations of P. rhoeas originating from Greece (9), Italy (5), France (10) and Spain (3). Plants resistant to the field rate of 2,4-D were observed in 25 of the 27 populations assayed, in frequencies ranging from 5% to 85%. Plants resistant to ALS-inhibiting herbicides (sulfonylureas) were present in 24 of the 27 populations, in frequencies ranging from 4% to 100%. Plants resistant to 2,4-D co-occurred with plants resistant to sulfonylureas in 23 populations. In four of these, the probability of presence of plants with cross- or multiple resistance to 2,4-D and sulfonylureas was higher than 0.5. ALS genotyping of plants from the field populations or of their progenies, identified ALS alleles carrying a mutation at codon Pro197 or Trp574 in 2,4-D-sensitive and in 2,4-D-resistant plants. The latter case confirmed multiple resistance to 2,4-D and ALS inhibitors at the level of individual plants in all four countries investigated. This study is the first to identify individual plants with multiple resistance in P. rhoeas, an attribute rarely assessed in other weed species, but one with significant implications in designing chemical control strategies.