BACKGROUND AND OBJECTIVE: Reltecimod, a CD 28 T-lymphocyte receptor mimetic, inhibits T-cell stimulation by an array of bacterial pathogens. A previous phase 2 trial demonstrated improved resolution of organ dysfunction after NSTI. We hypothesized that early administration of reltecimod would improve outcome in severe NSTI. METHODS: Randomized, double-blind, placebo-controlled trial of single dose reltecimod (0.5 mg/kg) administered within 6 hours of NSTI diagnosis at 65 of 93 study sites. Inclusion: surgical confirmation of NSTI and organ dysfunction [modified Sequential Organ Failure Assessment Score (mSOFA) score ≥3]. Primary analysis was modified Intent-to-Treat (mITT), responder analysis using a previously validated composite endpoint, necrotizing infection clinical composite endpoint, defined as: alive at day 28, ≤3 debridements, no amputation beyond first operation, and day 14 mSOFA ≤1 with ≥3 point reduction (organ dysfunction resolution). A prespecified, per protocol (PP) analysis excluded 17 patients with major protocol violations before unblinding. RESULTS: Two hundred ninety patients were enrolled, mITT (Reltecimod 142, Placebo 148): mean age 55 ± 15 years, 60% male, 42.4% diabetic, 28.6% perineal infection, screening mSOFA mean 5.5 ± 2.4. Twenty-eight-day mortality was 15% in both groups. mITT necrotizing infection clinical composite endpoint success was 48.6% reltecimod versus 39.9% placebo, P = 0.135 and PP was 54.3% reltecimod versus 40.3% placebo, P = 0.021. Resolution of organ dysfunction was 65.1% reltecimod versus 52.6% placebo, P = 0.041, mITT and 70.9% versus 53.4%, P = 0.005, PP. CONCLUSION: Early administration of reltecimod in severe NSTI resulted in a significant improvement in the primary composite endpoint in the PP population but not in the mITT population. Reltecimod was associated with improved resolution of organ dysfunction and hospital discharge status.