To reach the site of fertilisation and fertilise an oocyte, sperm must swim through the viscous fluids of the female tract, utilising adaptive motility as well as establishing capacitation and the acrosome reaction. All these processes are fuelled by ATP and thus require efficient ATP production along the entire sperm cell. Of relevance, we have recently identified an uncharacterised malate dehydrogenase (MDH1) paralogue, MDH1B, which is highly enriched in male germ cells and sperm. The canonical MDH1 is a cytoplasmic enzyme that translocates electrons generated during glycolysis into the electron transport chain for ATP payoff. To define the function of MDH1B we produced a knockout mouse model. Mdh1b-/- males were sterile but had histologically normal spermatogenesis and produced morphologically normal sperm. A key defect, however, was poor sperm motility arising from a stiff sperm midpiece, which reduced sperm tail amplitude (p < 0.001) and caused inefficient power dissipation from the tail (p < 0.01). When sperm from Mdh1b-/- were placed in viscous medium, we observed an inability to maintain rolling (3-dimensional) motility. Instead, sperm rapidly switched to a 2-dimensional, slithering motility and swam in circles. In accordance, oviduct dissection experiments revealed that Mdh1b-/- sperm were unable to reach the site of fertilisation in the oviductal ampulla following mating. Energy pathway analysis revealed a reduced capacity for oxidative phosphorylation, increased reliance on glycolysis, and a total reduction in ATP generation in Mdh1b-/- sperm, highlighting the importance of ATP generation along the entire sperm tail for sperm function. Both the precocious slithering motility and circular swimming defects were partially rescued by the addition of exogenous ATP. Collectively, these data reveal that MDH1B is an essential regulator of male fertility and suggest that MDH1B plays a vital role in the supply or production of ATP to power sperm motility through viscous fluids.