MR-guided radiotherapy enables real-time tracking of targets during therapy application, with the aim of improving targeting accuracy and, hence, treatment efficiency. To evaluate MR-based motion quantification techniques, a phantom that can generate reproducible, known motion is an extremely valuable tool. A modular motion phantom was designed and implemented that provides precise and reproducible linear motion, including accurate position information. The phantom was found to be MR-compatible. An initial evaluation of real-time bSSFP imaging was performed. The developed device can be used as research tool for MR-guided therapy and as a clinical quality assessment tool to improve treatment of patients.