The crystal structure and hydrogen positions of serandite, NaMn2[Si3O8(OH)], have been refined from single-crystal X-ray and time-of-flight neutron diffraction data at ambient conditions. The proton occupies an asymmetric, double-well position between O3 and O4, confirming one of the shortest asymmetric hydrogen bonds known in minerals with d(O3…O4) = 2.464(1) Å (X-ray) and 2.467(1) Å (neutron). The proton position closest to O3 has about 84% occupancy and an O-H distance of 1.078(3) Å, and the position closest to O4 has an occupancy of 16% and an O-H distance of 1.07(1) Å. The d(H…O) of these hydrogen bonds is 1.413(3) Å and 1.41(1) Å, respectively. Hydrogen bond angles are 164° for H1 and 168° for H2. The SiIV-OH bond length [1.628(1) Å] is intermediate in length among the three other Si-O bonds in the dominantly (84%) hydrated Si1 tetrahedron. These new structure data for a very strong hydrogen bond may be useful for extending spectroscopy-structure correlation diagrams into the region of very low energy O-H stretching.