Good quality Al2O3 thin films are deposited on polyethylene naphthalate (PEN) substrates through atomic layer deposition (ALD) at temperatures as low as 35 °C. Trimethylaluminum [TMA, Al(CH3)3] and water are used as precursors in the present study. Growth rates of 1.14 Å per cycle are observed at 35 °C, while the average arithmetic roughness (Ra) of the film is 0.86 nm. X-ray photoelectron spectroscopy (XPS) analysis confirms the high purity of the grown films with no carbon contamination. Good insulating properties are observed for the films and optical transmittance of more than 90% is recorded in the visible region.