Although mechanical weighing machines specialized for pigs are normally used to measure animal weights on pig farms, guiding those animals onto the weighing machines is a difficult and unpleasant chore that normally takes 20 seconds to complete after the pig is positioned on the load cell. Additionally, pig weighing machines are prone to mechanical breakdowns and are unpleasant to repair because of the farm residue they collect. Therefore, the development of a more practical and robust weight measurement apparatus is desired. In the present paper, we report on a handheld weight estimation device using a red-green-blue-depth (RGB-D) sensor with a laser slit system. An RGB-D sensor system is used to estimate pig weights, and the laser slit is used to align the measurement direction of the RGB-D sensor along the pig’s body. In operation, this system captures a depth image of the pig under examination and then uses it to create three-dimensional (3D) data. The 3D body parameters are extracted using image processing, and those extracted 3D body parameters for the pig are then fed into a random forest algorithm to produce the weight estimation. We also report on experimental results that demonstrate the reliability of our pig weight estimation system and its suitability for practical use.