Background The purpose of this systematic review and meta‐analysis was to determine the prevalence of trismus in patients with head and neck cancer. Methods Four electronic databases were searched: Cochrane Library, MEDLINE, EMBASE, and Web of Science up to April 2018. Level of evidence was assessed based on Oxford Centre for Evidence‐based Medicine. Publications were restricted to prospective cohort studies (n = 15), randomized clinical trials (n = 3), and cross‐sectional studies (n = 5) reporting on trismus as mouth maximum opening less than or equal to 35 mm in the English language. Results A total of 636 abstracts were reviewed for inclusion. Twenty‐three studies were included in qualitative analysis and 15 studies in meta‐analysis. The results included 2786 patients with head and neck cancer treated with surgery, radiotherapy, and/or chemotherapy either alone or in combination. Based on meta‐analyses, the weighted average prevalence of trismus increased from 17.3% at baseline (95% confidence interval [CI], 10.8%‐26.5%) to a peak of 44.1% at 6 months (95% CI, 36.7%‐51.8%) and decreased to 32.1% at 12 months (95% CI, 27.2%‐37.4%) and continued in average 32.6% at 3‐10 years (95% CI, 22.0%‐45.3%). Conclusions Having a primary site of the oral cavity or oropharynx was associated with a higher risk for developing trismus based on individual studies. Prospective cohort studies or randomized controlled trials with multiple objective trismus measurements, evaluation of disease characteristics, and treatment variables are needed to further investigate the prevalence of trismus secondary to head and neck cancer treatment.