AbstractTerrestrial and sub-Neptune planets are expected to form in the inner (less than 10 au) regions of protoplanetary disks¹. Water plays a key role in their formation^2–4, although it is yet unclear whether water molecules are formed in situ or transported from the outer disk^5,6. So far Spitzer Space Telescope observations have only provided water luminosity upper limits for dust-depleted inner disks⁷, similar to PDS 70, the first system with direct confirmation of protoplanet presence^8,9. Here we report JWST observations of PDS 70, a benchmark target to search for water in a disk hosting a large (approximately 54 au) planet-carved gap separating an inner and outer disk^10,11. Our findings show water in the inner disk of PDS 70. This implies that potential terrestrial planets forming therein have access to a water reservoir. The column densities of water vapour suggest in-situ formation via a reaction sequence involving O, H₂ and/or OH, and survival through water self-shielding⁵. This is also supported by the presence of CO₂ emission, another molecule sensitive to ultraviolet photodissociation. Dust shielding, and replenishment of both gas and small dust from the outer disk, may also play a role in sustaining the water reservoir¹². Our observations also reveal a strong variability of the mid-infrared spectral energy distribution, pointing to a change of inner disk geometry.