Organic semiconductor-based thin-film transistors (TFTs) have been extensively studied for organic electronics. In this study, we report on the influence of the polymer gate dielectric viscoelastic properties on overlying organic semiconductor film growth, film microstructure, and TFT response. From the knowledge that nanoscopically-confined thin polymer films exhibit glass-transition temperatures that deviate substantially from those of the corresponding bulk materials, we show here that pentacene (p-channel) and cyanoperylene (n-channel) films grown on polymeric gate dielectrics at temperatures well-below their bulk glass transition temperatures [T-g(b)] exhibit morphological/microstructural transitions and dramatic OTFT performance discontinuities at well-defined temperatures [associated with a polymer "surface glass transition temperature," or T-g(s)]. These transitions are characteristic of the particular polymer architecture and independent of film thickness or overall film cooperative chain dynamics. Our results demonstrate that TFT measurements represent a new and sensitive methodology to probe polymer surface viscoelastic properties.