Blossom is an open-source social robotics platform responding to three challenges in human-robot interaction (HRI) research: (1) Designing, manufacturing, and programming social robots requires a high level of technical knowledge; (2) social robot designs are fixed in appearance and movement capabilities, making them hard to adapt to a specific application; and (3) the use of rigid mechanisms and hard outer shells limits the robots’ expressive capabilities. Addressing these challenges, Blossom aims at three design objectives: accessibility, flexibility, and expressiveness. The robot’s mechanism can be quickly assembled and partially extended by end-users. Blossom’s appearance is open-ended through handcrafted fabric exteriors created and customized by users. Smooth organic movements are achieved with tensile mechanisms, elastic components, and a soft exterior cover attached loosely to the body. Blossom’s smartphone-based gesture generation requires neither programming nor character animation experience, allowing lay users to create their own behaviors. All elements in the design were conceived with a low barrier-of-entry in mind. The result is an accessible and customizable social robot for researchers. This article details the implementation of Blossom’s design and demonstrates the platform’s potential through four field deployment case studies.