Kepler-289 is a three-planet system exhibiting Transit Timing Variations (TTVs). Kepler-289 contains two sub-Neptunes and one gas giant planet orbiting a young, Sun-like star whose light curve exhibits significant variability due to rotational modulation from starspots. All three planets orbit within 0.52 AU of the host star with orbital periods just below a 1:2:4 Laplace orbital resonance. We observe a transit of Kepler-289c with the WIRC instrument on the 200” Hale Telescope at Palomar Observatory, using diffuser-assisted photometry to achieve space-like photometric precision from the ground. This new transit observation extends the original four-year Kepler TTV baseline by an additional 6.5 years. We re-reduce the archival Kepler data with an improved stellar activity correction and carry out a joint fit with the Palomar data to constrain the transit shape and derive updated transit times. We then model the TTVs to determine the masses of the three planets and constrain their densities. Our new analysis results in improved mass and density estimates for all three planets, with the innermost planet showing the largest improvement.