Trismus is characterized by limited mouth opening (<35 mm) that may be associated with pain. It is one of the most frequent complications of head and neck cancer treatment (5–38% of patients treated with extensive surgery and adjuvant radiation or exclusive radiotherapy). It usually occurs 3–6 months after radiotherapy is completed with the potential to increase its severity up to 2 years. Physiotherapy with trismus devices is often prescribed, using stretching techniques to increase the range of mouth opening. Recent studies have indicated positive results in terms of pain reduction and increased mouth opening for patients treated with photobiomodulation as an alternative or in complementation to physiotherapies.