Objective The systematic review evaluated the association of subclinical hypothyroidism (SCH) with metabolic syndrome (MetS) and specific MetS components in people with major psychiatric disorders. Methods A systematic review and meta-analysis was conducted to evaluate the association of SCH with MetS and its components in people with major psychiatric conditions. Results Five studies incorporating 24,158 participants met the inclusion criteria. All five studies comprised patients with depression and/or anxiety. Three studies incorporating 3365 participants were suitable for the meta-analysis. The pooled Odds Ratio (OR) of MetS was 3.46 (95% Confidence Interval/CI = 1.39–8.62) in major depressive disorder (MDD) and anxiety disorders patients with concurrent SCH compared to those without SCH. Meta-analysis showed a significant positive association between SCH and high body mass index (OR = 2.58, 95%CI = 1.33–5.01), high fasting plasma glucose (OR = 3.05, 95%CI = 1.79–5.18) and low high-density lipoprotein cholesterol (OR = 2.30, 95%CI = 1.82–2.92). Conclusions These findings suggest a significant positive association between MetS and SCH in people with MDD and anxiety disorders. This review informed the clinical implications of MetS in MDD with comorbid SCH and the importance of early diagnosis and treatment for SCH and MetS in psychiatric patients.