In plants, the hydroxymethylpyrimidine (HMP) and thiazole precursors of thiamin are synthesized and coupled together to form thiamin in plastids. Mutants unable to form HMP can be rescued by exogenous HMP, implying the presence of HMP transporters in the plasma membrane and plastids. Analysis of bacterial genomes revealed a transporter gene that is chromosomally clustered with thiamin biosynthesis and salvage genes. Its closest Arabidopsis homolog, the plastidic nucleobase transporter (PLUTO), is co-expressed with several thiamin biosynthetic enzymes. Heterologous expression of PLUTO in Escherichia coli or Saccharomyces cerevisiae increased sensitivity to a toxic HMP analog, and disrupting PLUTO in an HMP-requiring Arabidopsis line reduced root growth at low HMP concentrations. These data implicate PLUTO in plastidial transport and salvage of HMP.