Defect　engineering　in　photocatalysts　represents　a　fundamental　method　toward　tailoring　their　solar-to-chemical　energy　conversion　performance,　although　determining　the　nature　and　impact　of　subsurface　defects　remains　challenging.　Single-unit-cell　Bi2WO6　monolayers,　forming　a　sandwich-like　structure,　[BiO]+-[WO4]2--[BiO]+,　exhibit　promising　photocatalytic　performance　and　are　an　ideal　system　for　isolating　subsurface　defects.　We　report　the　single-step　synthesis　of　Bi2WO6　monolayers　rich　in　stable　interior　W　vacancies　and　characterize　their　influence　on　the　physical　properties　necessary　for　effective　photocatalytic　surface　reactions.　Defect-rich　monolayers　benefit　from　enhanced　visible-light　absorption　and　photocarrier　transport,　boosting　the　solar　photocatalytic　oxidation　of　benzylic　alcohols　by　140%　at　no　cost　to　selectivity　or　stability.　This　work　highlights　the　importance　of　subsurface　defects　within　surface-driven　photocatalytic　applications　and　prescribes　a　general　strategy　for　their　isolated　study　via　2D　compounds　exhibiting　symmetric　surface　termination.　©　2019　American　Chemical　Society.