At ambient pressure the crystal structure of L-serine monohydrate (L-serine monohydrate-I) contains H-bonded layers of zwitterionic serine molecules linked by H-bonds to water molecules. The waters act as donors to oxygen atoms on carboxylate and alcohol groups in separate layers. This phase remains stable from ambient pressure to 4.5 GPa; the most prominent structural change in this range is a reduction in the interlayer distance. On increasing the pressure to 5.2 GPa the structure transforms to a high-pressure polymorph, termed L-serine monohydrate-II. The structures of both polymorphs have been determined using a combination of X-ray single-crystal and neutron powder diffraction. During the transition the serine interlayer distances reduce further and the water molecules rotate so that both donor interactions are made to the same serine layer. The serine ammonium group adopts an eclipsed conformation, reconfiguring the H-bonding within the serine layers. The disruption of H-bonding as water is pushed into the serine layers suggests that a similar process may occur as a first step in the pressure-denaturation of proteins. Though the water molecules become coordinatively saturated with respect to H-bonding, and interlayer serine-serine Coulombic interactions are strengthened, PIXEL calculations show that overall the intermolecular interactions are weaker in phase-II than phase-I. The lattice enthalpy becomes more negative through the transition as a result of the smaller PV term applying to the more efficiently packed phase-II structure.