Glutathione S-transferase (GST) is involved in the downstream steps of the anthocyanin biosynthesis pathway in plants. However, the gene(s) encoding this enzyme have not been isolated from apple (Malus × domestica Borkh.) yet. We isolated a gene encoding GST from leaves of the red-fleshed apple variety ‘Royalty’ by full cDNA library sequencing and the 3′ rapid-amplification of cDNA ends method, and designated it MdGST1. In total, seven different MdGST1 transcripts were found. These had three different untranslated but identical protein-coding regions. Phylogenetic analysis showed that MdGST1 is a TT19-type GST, which is involved in anthocyanin transport. qRT-PCR analyses showed that the transcript level of MdGST1 was much higher in red leaves than in bagged or green leaves. When MdGST1 was introduced into a non-pigmented mutant of Arabidopsis, the transformants showed a visible purple phenotype in leaves and stems. Our results suggest that MdGST1 plays a role in anthocyanin biosynthesis. This information will help to improve the understanding of the mechanism of apple coloration.