Aims: To determine whether prophylactic calibrated jaw stretching using Restorabite® prevents trismus in patients with normal mouth opening prior to head and neck cancer (HNC) treatment, and whether exercise dose differentially affects maximal incisal opening (MIO). Methodology: Prospective RCT across 4 Australian centres. Participants with HNC and pre-treatment MIO ≥ 35 mm were randomised 2:1 to Restorabite® or stacked tongue depressors for 10 weeks, with Restorabite® participants further allocated to high- or low-pressure protocols. Primary outcome was MIO change at 10 weeks; secondary outcomes included the Gothenburg Trismus Questionnaire (GTQ). ANCOVA adjusted for baseline MIO and treatment modality. Results: 93 of 97 recruited adults completed the intervention (4% withdrawal). Median baseline MIO was 45 mm (IQR 39–50). Restorabite® (n = 60) versus control (n = 33): greater MIO preservation (+2.15 mm vs −0.62 mm; adjusted β = 2.77 mm; 95% CI 0.58–4.95; p = 0.014). High- versus low-pressure: superior MIO outcomes with high-pressure (+4.10 mm vs +0.07 mm; adjusted β = 3.39 mm; 95% CI 0.50–6.28; p = 0.022). Trismus incidence at 10 weeks: 7% versus 15%. No significant GTQ differences. No serious adverse events. Conclusion: Restorabite® significantly attenuates MIO decline during HNC treatment, with high-pressure exercise producing superior outcomes consistent with a dose-response relationship. These findings support Restorabite® as a prophylactic mechanotherapy device in patients at risk of treatment-related trismus.