Trismus, or restricted mouth opening is a very common and often neglected side effect in oral and oropharyngeal malignancies owing to the disease itself as well as its treatment with surgery and/or chemoradiation. This study aims at assessing the prevalence of trismus occurring after complete treatment in diagnosed cases of oral and oropharyngeal malignancy and to assess various risk factors associated with development of trismus in these patients. A prospective, observational study conducted over a 2 years at a tertiary care hospital. All newly diagnosed cases of oral and oropharyngeal malignancy seeking treatment were included. Maximal mouth opening as the interincisoral distance measured using a Vernier Calliper was done so in each patient at presentation before initiation of treatment, before adjuvant therapy, 1 month, 3 months and 6 months post treatment and the results were analysed. The primary modality of treatment, type of surgery, reconstruction performed and patient's recovery were monitored. Out of 72 patients included, 60 had oral cancers and the remaining oropharyngeal. The overall prevalence was 61.1%. The following variables were found to be associated with higher prevalence of trismus- Tumour size, advanced primary disease (T3, T4 disease), oral submucous fibrosis and radiation. There is a high prevalence of trismus in treated patients of oral and oropharyngeal malignancies more so in cases of oral cancer undergoing surgery with adjuvant therapy. There is a need to find treatment modalities with better functional outcomes and direct oral rehabilitation strategies towards alleviation of trismus.