Canterbury bells (Campanula medium) have deep purple petals due to the accumulation of 7-polyacylated anthocyanin molecules. The first step in the production of 7-polyacylated anthocyanins is glucosylation at the C7 position of anthocyanidin mediated by an acyl-glucose dependent anthocyanin 7-O-glucosyltransferase (AA7GT). To date, two such enzymes have been identified: DgAA7GT from delphinium (Delphinium grandiflorum) and AaAA7GT from African lily (Agapanthus africanus). Here, we describe the isolation of AA7GT cDNA from C. medium and the characterization of the enzymatic properties of a recombinant protein. The CmAA7GT protein belongs to glycoside hydrolase family 1, similarly to other AA7GTs; a phylogenetic analysis revealed that CmAA7GT was in the same clade as other AA7GTs. The CmAA7GT gene showed expression only in flowers, with a peak level of expression at the middle stage of floral development. A recombinant CmAA7GT protein showed significant preference for interaction with anthocyanidin 3-O-rutinoside rather than anthocyanidin 3-O-monoglycoside, which is the preferred target of other AA7GTs. This difference in target preference may reflect a conformational difference in the acceptor pocket of the enzyme protein that recognizes the anthocyanidin glycoside.