We here propose a highly-sensitive, simple moisture sensor based on surface plasmon resonance. The moisture sensor was constructed by coating a hydrogel film onto a stainless steel (STS) plate having a one-dimensional surface texture with a pitch of 500 nm. The refractive index of the hydrogel film was modulated upon exposure to moisture, shifting the plasmonic resonance peak of the textured STS surface. The effective refractive index and water content of the film were deduced from the spectral shift of the resonance peak. The resonance position shifted from 748 to 720 nm as the water volume percentage increased from 48.8 to 86.6 vol%, corresponding to a moisture sensitivity of 0.74 nm per 1 vol%. The refractive-index sensitivity of the sensor was measured to be 593 nm/RIU, where RIU is refractive index unit.