To develop a growing root, cell division in the root meristem has to be properly regulated in order to generate or propagate new cells. How cell division is regulated in the root meristem remains largely unknown. Here, we report the identification and characterization of the Arabidopsis (Arabidopsis thaliana) RETARDED ROOT GROWTH (RRG) gene that plays a role in the regulation of root meristem cell division. In the root, RRG is predominantly expressed in the root meristem. Disruption of RRG function reduced numbers of dividing cells, the rate of cell production, and endoreduplication, and thus affected meristem size and root growth. Quantitative reverse transcription-polymerase chain reaction and marker-assisted analyses revealed that expression levels of several cell cycle genes were decreased in the mutant roots, indicating a defect in cell cycle progression. Mutations in RRG, however, did not affect the expression of key root-patterning genes and an auxin-responsive marker, suggesting that RRG is not essential for root patterning and auxin signaling. RRG is a mitochondria-localized protein conserved in plants and shares a DUF155 domain with proteins related to cell division in yeast, and rrg mutants displayed extensive vacuolization in mitochondria. We propose that Arabidopsis RRG is a conserved mitochondrial protein required for cell division in the root meristem.