We investigated the impact of pressure on thermophilic, chemolithoautotrophic NO3− reducing bacteria of the phyla Campylobacterota and Aquificota isolated from deep-sea hydrothermal vents. Batch incubations at 5 and 20 MPa resulted in decreased NO3− consumption, lower cell concentrations, and overall slower growth in Caminibacter mediatlanticus (Campylobacterota) and Thermovibrio ammonificans (Aquificota), relative to batch incubations near standard pressure (0.2 MPa) conditions. Nitrogen isotope fractionation effects from chemolithoautotrophic NO3− reduction by both microorganisms were, on the contrary, maintained under all pressure conditions. Comparable chemolithoautotrophic NO3− reducing activities between previously reported natural hydrothermal vent fluid microbial communities dominated by Campylobacterota at 25 MPa and Campylobacterota laboratory isolates at 0.2 MPa, suggest robust similarities in cell-specific NO3− reduction rates and doubling times between microbial populations and communities growing maximally under similar temperature conditions. Physiological and metabolic comparisons of our results with other studies of pressure effects on anaerobic chemolithoautotrophic processes (i.e., microbial S0-oxidation coupled to Fe(III) reduction and hydrogenotrophic methanogenesis) suggest that anaerobic chemolithoautotrophs relying on oxidation–reduction (redox) reactions that yield higher Gibbs energies experience larger shifts in cell-specific respiration rates and doubling times at increased pressures. Overall, our results advance understanding of the role of pressure, its relationship with temperature and redox conditions, and their effects on seafloor chemolithoautotrophic NO3− reduction and other anaerobic chemolithoautotrophic processes.