Polarons are ubiquitous in many semiconductors and have been linked with conductivity and op-
tical response of materials for photovoltaics and heterogeneous catalysis, yet how surface polarons
influence adsorption remains unclear. Here, by modelling the surface of rutile titania using density
functional theory, we reveal the effect of small surface polarons on water adsorption, dissociation,
and hydrogen bonding. On the one hand the presence of such polarons significantly suppresses
dissociation of water molecules that are bonded directly to polaronic sites. On the other hand, po-
larons facilitate water dissociation at certain non-polaronic sites. Furthermore, polarons strengthen
hydrogen bonds, which in turn affects water dissociation in hydrogen bonded overlayer structures.
This study reveals that polarons at the rutile surface have complex, multi-faceted, effects on water
adsorption, dissociation and hydrogen bonding, highlighting the importance of polarons on water
structure and dynamics on such surfaces. We expect that many of the physical properties of surface
polarons identified here will apply more generally to surfaces and interfaces that can host small
polarons, beyond titania.