Flat metasurfaces with subwavelength meta-atoms can be designed to manipulate the electromagnetic parameters of incident light and enable unusual light-matter interactions beyond the capabilities of the constituent materials themselves. Although hydrogel-based metasurfaces have the potential to control optical properties dynamically in response to environmental conditions, the pattern resolution of these surfaces has typically been limited to microscale features or larger, limiting capabilities at the nanoscale and precluding effective use in metamaterials. This paper reports a general approach to developing reversibly tunable plasmonic metasurfaces with hydrogel meta-atoms at the subwavelength scale. Periodic arrays of hydrogel nanodots with continuously tunable diameters between 180 nm and 240 nm were fabricated on silver substrates, resulting in humidity-responsive surface plasmon polaritons (SPPs) at the nanostructure-metal interfaces. The peaks of the SPPs were controlled reversibly by absorbing or releasing water within the hydrogel matrix; the matrix subsequently generated plasmonic color rendering in the visible spectrum. We demonstrated that metasurfaces designed with these spatially patterned nanodots of varying sizes can benefit applications in anti-counterfeiting and generate multicolored displays with single-nanodot resolution. Furthermore, we showed system versatility exhibited by broadband beam-steering on a phase modulator consisting of hydrogel supercell units in which the size variations of constituent hydrogel nanostructures engineer the wavefront of reflected light from the metasurface.