The basic components and mechanisms of mitochondrial transcription in mammals have been described, however, the components involved in mRNA processing, translation and stability remain largely unknown. In plants, pentatricopeptide domain RNA-binding proteins regulate the stability, expression and translation of mitochondrial transcripts. Here, we investigated the role of an uncharacterized mammalian pentatricopeptide domain protein, pentatricopeptide repeat domain protein 3 (PTCD3), and showed that it is a mitochondrial protein that associates with the small subunit of mitochondrial ribosomes. PTCD3 knockdown and over expression did not affect mitochondrial mRNA levels, suggesting that PTCD3 is not involved in RNA processing and stability. However, lowering PTCD3 in 143B osteosarcoma cells decreased mitochondrial protein synthesis, mitochondrial respiration and the activity of Complexes III and IV, suggesting that PTCD3 has a role in mitochondrial translation.