Signal Transducer and activator of transcription (STAT) 3 is key signaling molecule activated by growth factors and cytokines to regulate proliferation, immunity and metabolism. In addition to the vital roles played by STAT3 in normal tissue development and homeostasis, aberrant STAT3 activity has been linked to more than 50% of all human cancers. Canonical STAT3 signaling is dependent on phosphorylation of a carboxyl terminal tyrosine (Y705) which facilitates dimerization, DNA binding and transcriptional activation of thousands of target genes. Recently a pool of STAT3 was also found in the mitochondria where it enhances the activity of Complex I, II and V of the electron transport chain (ETC). The mitochondrial activity of STAT3 is dependent on phosphorylation on serine 727 (S727) and independent of Y705 phosphorylation. Whilst STAT3 can physically interact with complex I, the relative concentration of STAT3 is vastly less than complex I proteins indicating that this direct protein-protein interaction is unlikely to be responsible for modulation of ETC activity. Using size exclusion chromatography, mass spectrometry and co-immunoprecipitation we found that mitochondrial STAT3 forms a stable protein complex approximately 400 kDa, with Leucine Rich Pentatricopeptide Repeat Containing protein (LRPPRC) and Stem-Loop Interacting RNA Binding Protein (SLIRP), which are required for mitochondrial RNA stability and translation respectively. Knock out of STAT3 caused a 2 fold decrease in poly adenylated mitochondrial RNA abundance and decreased the rate of mitochondrial translation. These findings suggest novel roles of STAT3 which involves regulation of RNA stability and translational kinetics within mitochondria.