Due to the development of depth sensors, such as time-of-flight (ToF) cameras, it becomes easier to acquire depth information directly from a scene. Although such devices enable us to obtain depth maps at video frame rates, the depth maps often have low resolutions only. A typical ToF camera retrieves depth maps of resolution 320 × 200, which is much lower than the resolutions of high definition color images. In this work, we propose a depth image super-resolution algorithm, which operates robustly even when there is a large resolution gap between a depth image and a reference color image. To prevent edge smoothing artifacts, which are the main drawback of conventional techniques, we adopt a superpixel-based approach and develop an edge enhancing scheme. Simulation results demonstrate that the proposed algorithm aligns the edges of a depth map to accurately coincide with those of a high resolution color image.