Here, we report for the first time that resonance in dynamic-mode cantilever sensors persists in hydrogels and enables the real-time characterization of hydrogel viscoelastic properties and the continuous monitoring of sol-gel phase transitions (i.e., gelation and dissolution processes). Real-time tracking of piezoelectric-excited millimeter cantilever (PEMC) sensor resonant frequency (f(air)=55.4 +/- 8.8kHz; n=5 sensors) and quality factor (Q; Q(air)=23.8 +/- 1.5) enabled continuous monitoring of high-frequency hydrogel shear storage and loss moduli (G ' (f) and G '' (f), respectively) calculated by sensor data and fluid-structure interaction models. Changes in the sensor phase angle, quality factor, and high-frequency shear moduli obtained at the resonant frequency (G ' (f) and G '' (f)) correlated with low-frequency moduli obtained at 1Hz using dynamic mechanical analysis. Characterization studies were performed using physically and chemically crosslinked hydrogel systems, including gelatin hydrogels (6-10wt.%) and alginate hydrogels (0.25-0.75wt.%). The sensor exhibited a dynamic range from the rheological properties of inviscid solutions to hydrogels with high-frequency moduli of 80kPa and low-frequency moduli of 26kPa. The sensor exhibited a limit of detection of 260Pa and 1.9kPa for changes in hydrogel storage modulus (E ') based on the sensor's phase angle and quality factor responses, respectively. We also show that sensor data enable quantitative characterization of gelation process dynamics using a modified Hill model. This work suggests that cantilever sensors provide a promising platform for the sensor-based characterization of hydrogels, such as quantification of viscoelastic properties and real-time monitoring of gelation processes.