*Red rice, a major agricultural weed, is phenotypically diverse and possesses traits that are similar to both wild and cultivated rice. The genetic resources available for rice make it possible to examine the molecular basis and evolution of traits characterizing this weed. Here, we assess the phenol reaction - a classical trait for distinguishing among cultivated rice varieties - in red rice at the phenotypic and molecular levels. *We phenotyped more than 100 US weed samples for the phenol reaction and sequenced the underlying Phr1 locus in a subset of samples. Data were analyzed in combination with previously published Phr1 data for cultivated rice. *Most weed accessions (96.3%) are positive for the phenol reaction, and samples with a negative response carry loss-of-function alleles that are rare or heretofore undocumented. One such allele may have evolved through mutational convergence of a 1-bp frameshift insertion. Haplotype sharing between red rice and US cultivars suggests occasional crop-weed hybridization. *Our discovery of previously undocumented nonfunctional phr1 alleles suggests that there are likely to be other loss-of-function mutations segregating in Oryza sativa around the world. Red rice may provide a useful study system for understanding the adaptive significance of Phr1 variation in agricultural settings.