Background: Periodontal disease represents a global public health concern, with a disproportionate burden being borne by vulnerable populations. One such group is people with severe mental illness (SMI), and this study examined whether periodontal health is poorer in people with SMI than the general population. Methods: We conducted a systematic search for studies published before March 2021 on the periodontal health of people with SMI using the following databases: PubMed, PsycINFO, EMBASE, China National Knowledge Infrastructure, and Chongqing VIP. Outcomes were periodontitis, periodontal disease, and shallow and deep periodontal pockets. Results were compared with the general population. Results: Seventeen studies had sufficient data for a random-effects meta-analysis, consisting of 4404 psychiatric patients and 95,411 controls. SMI was associated with an increased prevalence of periodontitis (odds ratio = 1.97, 95% confidence interval [CI] = 1.15-3.35) compared with the general population. People with SMI had 4.28 the odds of having periodontal disease compared with controls (95% CI = 2.54-7.21). They also had 3.65 the odds of shallow pockets (95% CI = 1.80-7.42) and 2.76 the odds of deep pockets (95% CI = 1.10-6.93). Conclusions: Our findings highlight the increased prevalence and severity of periodontal disease in people with SMI. Oral health is often considered the gateway to overall health and should be a public health priority for this population.