BACKGROUND: Delirium can occur in patients with pneumonia, but its prevalence is inconsistent across studies. Unreliable estimates and uncertainty regarding the significance of patient-specific vs. microbiological risk factors hinder delirium management and prognosis. Here, we provide robust estimates of delirium prevalence in patients with pneumonia, associated risk factors, and association with mortality. METHODS: We searched five databases (MEDLINE, Cochrane Library, Embase, PsycINFO, and Scopus), from inception to August 6, 2024. We included studies in adults hospitalized with pneumonia reporting delirium, encephalopathy, or altered mental status. Two investigators extracted data and assessed risk of bias. Summary rates were calculated using random-effects models. We performed prespecified analyses for diagnostic methods, microbiologic factors, clinical factors, and mortality, with sensitivity analysis among studies at low risk of bias. Registration: PROSPERO-CRD42023385571. RESULTS: Delirium prevalence across 126 studies was 22% (95% CI [18%-26%]), and higher in studies at low risk of bias (40% [24%-58%], n=11). Standardized assessments yielded higher rates than symptom- or ICD code-based assessments (p<0.05). Surprisingly, delirium rates did not differ by microbiological etiology (p0.63), including COVID-19, nor by pneumonia origin (p=0.14). Predisposing factors included older age and neurologic and systemic comorbidities. Delirium was associated with increased mortality (OR 4.3 [3.24-5.76], p<0.001), without change over five decades (p = 0.32). INTERPRETATION: Delirium is highly prevalent and enduring in pneumonia. Our results emphasize patient- and care-related factors over microbiological causes, including COVID-19. Delirium's entrenched association with mortality, even considering covariates, reinforces the need to manage delirium as a convergent syndrome in pneumonia.