We modulated the grafting density (sigma) and chain length of polystyrene (PS) brushes on substrates to probe their effect on the glass transition temperature (T-g) in overlaying PS thin films. The T-g of PS films was analyzed as a function of brush thickness (or sigma R-g(2) ), where R-g is the radius of gyration of PS brushes. Our results indicate that PS films below 90 nm exhibit a nonmonotonic dependence of T-g on grafting density with the maximum T-g occurring near the regime of wetting-dewetting transition of the overlaying melt. The maximum T-g was found to be higher than the value of bulk PS system. Computer simulation results suggest that such trends arise as a consequence of the environment of enhanced friction presented by the brush layer and the overlap between the polymer film and the grafted layer.