Many heat-shock proteins (Hsp) are members of evolutionarily conserved families of chaperone proteins that inhibit the aggregation of unfolded polypeptides and refold denatured proteins, thereby remedying phenotypic effects that may result from protein aggregation or protein instability. Here we report that the mitochondrial chaperone Hsp40, also known as Dnaja3 or Tid1, is differentially expressed during cardiac development and pathological hypertrophy. Mice deficient in Dnaja3 developed dilated cardiomyopathy (DCM) and died before 10 weeks of age. Progressive respiratory chain deficiency and decreased copy number of mitochondrial DNA were evident in cardiomyocytes lacking Dnaja3. Profiling of Dnaja3-interacting proteins identified the alpha-subunit of DNA polymerase gamma (Polga) as a client protein. These findings suggest that Dnaja3 is crucial for mitochondrial biogenesis, at least in part, through its chaperone activity on Polga and provide genetic evidence of the necessity for mitochondrial Hsp40 in preventing DCM.