In this paper, for the first time frequency tuning in lateral-mode thin-film piezoelectric resonators is demonstrated through varying the termination load connected to an isolated tuning port. The dependency of frequency tuning on the design features of the resonator is studied as well. More than 4300ppm tuning range in a thin-film piezoelectric-on-silicon (TPoS) resonator and more than 9000ppm tuning range in a pure piezoelectric resonator is achieved at ~29.5MHz and ~31.5MHz respectively. Also, an oscillator tuning range of more than 2700ppm is reported which shows a great potential for temperature compensated oscillators.