This work presents the design and development of an assistive device to enable natural sit-to-stand (STS) motion for individuals with lower extremity weakness. The device is conceptualized based on the synthesis of a four-bar linkage, the coupler of which follows the elbow trajectory of the user during sit-to-stand motion. The four-bar mechanism is designed and synthesized using the Burmester curve theory. In this work, two sit-to-stand assistive devices are designed, fabricated, and developed. The first device has a front tray for multipurpose work. Besides elbow support, it can be used as a laptop placement, for eating purposes, etc. The elbow support of the STS device with the tray rotates by 5 degrees. In the foldable STS assistive device, the elbow support is mounted on a parallelogram mechanism, which remains horizontal during the STS motion. A scissors mechanism is used to incorporate the foldability feature into the device. Both of the devices are portable and actuated by battery-operated linear actuators. These developed assistive devices are simple to use, lightweight, and modular for easy assembly. An optimization technique is formulated to predict assistive sit-to-stand movements of the human body and calculate the support force during STS motion. The effectiveness of the device is demonstrated by testing on humans, showing a significant reduction in ground reaction force (GRF) during STS motion of around 45% and a commensurate reduction in the user’s effort for STS. The device’s straightforward and user-friendly design makes it a promising tool for people with lower extremity weaknesses to enhance their general well-being. It is a potentially useful tool for increasing mobility and raising the quality of life for those with lower extremity weakness. The technology can be improved and tailored for broader therapeutic uses through further study that builds on current discoveries.