Abstract Background Atopic dermatitis (AD) is an inflammatory skin disease with significant unmet need. Blockade of the OX40–OX40 ligand (OX40L) costimulation pathway by targeting OX40L on antigen-presenting cells (APCs) with a fully human noncytotoxic, nondepleting anti-OX40L monoclonal antibody (amlitelimab; SAR445229; KY1005) is a novel way to modulate persistent inflammation. Objectives To assess the safety and efficacy of amlitelimab over 16 weeks in adults with AD in a phase IIa double-blind placebo-controlled study. Methods The study was conducted at 19 hospitals in Germany, Poland, Spain and the UK. Eligible patients with moderate-to-severe AD were randomized (1 : 1 : 1) to low-dose intravenous (IV) amlitelimab (200 mg), high-dose IV amlitelimab (500 mg) or placebo, followed by three maintenance doses (50% of loading dose) at 4, 8 and 12 weeks, with safety follow-up to week 36. The co-primary endpoints were the incidence of treatment-emergent adverse events (all patients who received ≥ 1 dose of the study drug) and mean percentage change in Eczema Area and Severity Index (EASI) to week 16 (full analysis set). Results Between 13 December 2018 and 12 May 2020, 89 patients were randomly assigned to low- (n = 29) or high-dose amlitelimab (n = 30) or placebo (n = 29), of whom 88 proceeded to treatment [37 women (42%), 51 (58%) men; mean (SD) age 33.6 (11.9) years]. Amlitelimab was generally well tolerated with an unremarkable safety profile; no hypersensitivity events were reported. For the primary endpoint, the least square mean percentage change in EASI from baseline to week 16 was –80.12% [95% confidence interval (CI) –95.55 to –64.68; P = 0.009 vs. placebo] and –69.97% (95% CI –85.04 to –54.60; P = 0.07 vs. placebo) for the low- (n = 27) and high-dose (n = 27) amlitelimab groups, respectively, vs. –49.37% (95% CI –66.02 to –32.72) for placebo (n = 24). Numerically greater reductions in EASI were observed for amlitelimab vs. placebo from weeks 2 to 16. Conclusions Novel targeting of OX40L-expressing APCs with amlitelimab was well tolerated and resulted in clinically meaningful improvements in AD.