We present polydimethylsiloxane (PDMS) microlenses capable of thermally tuning their focal lengths via optically transparent indium tin oxide (ITO) microheaters. Microlenses with various diameters are created by molding PDMS over microstructures of reflowed photoresist. Due to a relatively large thermal expansion coefficient, these PDMS microlenses exhibit measurable thermal expansion with relatively small temperature changes. By adjusting the temperature of the microlenses using ITO microheaters, we can actively control their radii of curvature and focal lengths without requiring moving mechanical components. To demonstrate the tunability of the microlenses, we characterize temperature-dependent changes by focal length and magnification factor measurements. We also investigate the ability to use the microlenses for tunable observation of cells, demonstrating their potential application to biological imaging.