We studied the interaction of lipid membranes with the deep-sea osmolyte trimethalamine-N-oxide (TMAO), which is known to stabilize proteins most efficiently against various environmental stress factors, including high hydrostatic pressure (HHP). Small-angle X-ray-scattering was applied in combination with fluorescence and infrared spectroscopy, calorimetric and AFM measurements to yield insights into the influence of TMAO on the supramolecular structure, hydration level, lipid order as well as the phase behavior of one- and three-component model biomembranes, covering a large region of the temperature-pressure phase space. Our results show that TMAO has not only a marked effect on the conformational dynamics and stability of proteins and nucleic acids, but also on lipid bilayer systems. The gel-to-fluid phase transition is shifted to higher temperatures with increasing TMAO concentration, and the lipid order parameter increases in the fluid lipid phase. Strong H-bonding with bulk water and preferential exclusion of TMAO from the lipid headgroup region leads to a drastic loss of water in the interlamellar space of fully hydrated multivesicular lipid assemblies. HHP leads to an increase of the lipid order parameter of fluid membranes as well, resulting in an increase of the lipid length. Such effect is rather small, however, and the marked effect TMAO imposes on the interlamellar spacing of the lipid bilayers is not significantly affected by temperature and high pressure. Furthermore, the lateral organization of heterogeneous model membranes changes upon addition of the cosolvent. TMAO leads to a coalescence of lipid domains, probably due to an increase of the line tension between liquid ordered and disordered domains in such raft-like lipid bilayer structures.