An innovative 3D optical crackmeter employing computer vision technology is used for displacement monitoring in a crack of a retaining wall automatically and remotely. The 3D optical crackmeter is composed of a Raspberry Pi device and a digital camera in a box, and a fixed chessboard on the two sides of a crack. A network with LoRa wireless communication can be connected as an IoT system to provide automatic remote functions. The OpenCV library is employed to analyze changes in chessboard imaging so that relative displacements of the crack in the retaining wall can be measured in a landslide area. Through laboratory and field testing, the resolution and accuracy of the 3D optical crackmeter were determined as 0.04 and 0.1 mm, respectively. Using the crackmeter, we observed significant displacements in the x- and z-directions of the crack in a retaining wall of 0.067 and 0.060 cm, respectively, in the Jhongsinlun landslide area of Taiwan over three months. Overall, the 3D optical crackmeter with computer vision technology can accurately measure the 3D displacement of cracks in a retaining wall. Moreover, the IoT-based 3D optical crackmeter is more cost-effective than traditional crackmeters used in landslide areas.