This study aimed to investigate the efficacy and safety of antitumor necrosis factor-alpha (TNF-α) therapy using adalimumab in patients with chronic schizophrenia. This is a randomized, double-blind, placebo-controlled clinical trial carried out at Roozbeh Hospital (Tehran, Iran) from June 2020 to October 2021. The patients were randomly divided into two parallel adalimumab + risperidone and placebo + risperidone groups. Participants in the intervention group received adalimumab subcutaneous injection (40 mg) by pen-injector at weeks 0 and 4. Using the Positive and Negative Symptoms Scale (PANSS), patients' positive and negative symptoms were assessed at weeks 0, 4, and 8. Forty patients (20 in each group) were included. PANSS total (t = 4.43, df = 38, P < 0.001), negative (t = 2.88, df = 38, P = 0.006), and general psychopathology (t = 4.06, df = 38, P < 0.001) scores demonstrated a significantly greater decline in adalimumab compared with the placebo group from baseline study endpoint. However, improvement of PANSS positive subscale scores showed no significant difference from the baseline study endpoint. There was no significant between-group difference regarding levels of C-reactive protein, interleukin (IL)-1β, TNF-α, IL-6, and IL-8 at baseline and also at the week 8 visit (P > 0.05 for all). The current study found adalimumab adjunctive therapy effective in treating schizophrenia, particularly its negative and general psychopathology symptoms, with no side effects.