Organisms are thriving in the deep sea at pressures of up to the 1 kbar level. To withstand such harsh conditions, they accumulate particular osmolyte mixtures to counteract the pressure stress imposed. We explored the combined effects of pressure and osmolyte mixtures known from deep sea organisms on the closed-to-open conformational transition of a DNA hairpin (Hp). To this end, single-molecule Förster resonance energy transfer (smFRET) experiments were carried out in an optimized high-pressure capillary optical cell. In the absence of osmolytes, pressure shifts the conformational equilibrium of the DNA Hp towards the open, unfolded state owing to a volume decrease of about −20 cm3 mol−1. We show that the deep-sea osmolyte mixture, largely composed of TMAO, is able to rescue the DNA Hp from unfolding even up to almost 1 kbar, which is supposed to be essentially due to a distinct excluded volume effect.