Background: Skull base osteomyelitis and necrotising otitis externa require prolonged antibiotic therapy, yet determining optimal treatment cessation timing remains challenging. Conventional imaging modalities demonstrate persistent abnormalities beyond infection resolution, confounding treatment decisions. This systematic review evaluated the diagnostic test accuracy of ¹⁸F-fluorodeoxyglucose positron emission tomography (¹⁸F-FDG PET) for treatment response monitoring in skull base osteomyelitis and necrotising otitis externa. Methods: We conducted a systematic review following PRISMA-DTA guidelines, searching MEDLINE, Embase, CENTRAL, CINAHL, Scopus, and Web of Science from inception to November 2025. Studies evaluating ¹⁸F-FDG PET diagnostic accuracy for treatment response assessment in confirmed skull base osteomyelitis or necrotising otitis externa were included. Two reviewers independently screened studies, extracted data, and assessed risk of bias. Bivariate random-effects meta-analysis was performed using MetaBayesDTA to obtain pooled sensitivity and specificity. Results: Eight studies comprising 154 lesions contributed to the primary analysis. Pooled sensitivity was 95.2% (95% credible interval (CrI) 85.6–99.0%) and pooled specificity was 89.1% (95%CrI 70.7–96.7%). The positive likelihood ratio was 8.7 (95%CrI 3.2–28.4) and negative likelihood ratio was 0.05 (95%CrI 0.01–0.17), with a diagnostic odds ratio of 172.0. Seven studies evaluating diagnostic positivity at initial presentation yielded a pooled rate of 96.1% (95% CI 91.3–98.3%). SUVmax was the most frequently used metabolic parameter. Conclusions: ¹⁸F-FDG PET, specifically using SUVmax, demonstrates high sensitivity and good specificity for treatment response monitoring, with excellent capacity to rule out persistent infection. However, evidence quality is limited by retrospective designs and heterogeneity. Prospective studies and standardised thresholds are needed to validate utility.