Trismus is a post-radiotherapy complication that affects survivors of head and neck cancers. It is described as the reduction of mouth opening to less than 35 mm. The condition is a progressive process; hence early intervention is crucial to prevent further damage and weakness to the elevator muscles of the mandible. Here, we report a 40-year-old woman with known squamous cell carcinoma of the maxillary sinus in stage T3N0M0 who had undergone upper right partial maxillectomy with subsequent 33 fractions of radiotherapy (66 Gy; intensity-modulating radiotherapy). Five months post-radiotherapy, the patient showed interest in replacing her missing teeth to restore her oral functions and facial appearance. At her post-radiotherapy visit in the dental assessment clinic, she showed a mouth opening of 12 mm. The TheraBite® Jaw Motion Rehabilitation System™ and wooden tongue depressors were alternatively used for 6 weeks (4–5 stretches/day; each stretch held for 15 s). Mouth opening gradually increased to 20 mm. Consequently, a definitive obturator prosthesis was fabricated using computer-aided design and computer-aided manufacturing technologies with the 3Shape D900 model scanner. These were challenging and time-consuming steps, but acceptable facial appearance and mouth opening were restored for the patient. This case suggests that Therabite® Jaw Motion Rehabilitation System TM and wooden tongue depressors help to increase mouth opening significantly if used together and alternatively.