In this paper, we describe the design, fabrication, and characterization of a flexible platinum temperature sensor embedded in polyimide (PI) films for temperature monitoring applications on curved surfaces, such as the skin of a human body, and various tubes and pipes. In order to optimize and enhance the properties of the flexible temperature sensors, we investigated the effects of the designed thickness (0.5, 1.0, and 1.5 µm) and length (10, 50, 100, and 150 mm) of Pt thin-film electrodes embedded in PI films on the temperature coefficient of resistance (TCR) properties and sensitivities. The average TCRs for flexible temperature sensors with different lengths but identical thicknesses of Pt thin-film electrodes were 0.0027, 0.0029, and 0.0030 ℃−1 for designed thicknesses of 0.5, 1.0, and 1.5 µm of Pt thin-film electrodes, respectively. The TCRs slightly increased when the thickness of the Pt electrodes increased. Meanwhile, thinner or longer Pt electrodes led to higher sensitivity for the temperature sensors. The highest sensitivity of a fabricated flexible temperature sensor was 15.59 Ω∙℃−1 in the device with a length of 150 mm and thickness of 0.5 µm. The bending effect on the change in the TCR of flexible temperature sensors, with radii of curvature of 5, 10, and 15 mm, was small. In order to confirm the performance of the fabricated flexible temperature sensor, actual skin temperatures of the palm and back of the hand were measured.