Aim To investigate the effect of theaflavin on oxidized low density lipoprotein (ox-LDL)-induced foam cell formation and oxidative stress in THP-1 macrophages and its mechanism. Methods THP-1 derived macrophages were pretreated with 50 μmol/L theaflavin and (or) 10 μmol/L nuclear factor erythroid 2-related factor 2 (NRF2) inhibitor ML385, then 100 mg/L ox-LDL was added to the cells for 24 h to establish the foam cell model. The effect of theaflavin on THP-1 macrophages viability was evaluated by CCK-8 assay and LDH release. The expression of inflammatory cytokines interleukin-6 (IL-6), interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α) were analyzed by real-time quantitative polymerase chain reaction (RT-qPCR) and Western blot. The release of inflammatory cytokines were detected by enzyme linked immunosorbent assay (ELISA). Intracellular lipid accumulation was detected by Oil red O staining, and lipid absorption was observed by DiL-labeled oxidized low density lipoprotein (DiL-ox-LDL) staining. Reactive oxygen species (ROS) level was detected by DCFH-DA probe. The expression of lipid uptake, cholesterol efflux and oxidative stress-related proteins were detected by Western blot and RT-qPCR. Results Treatment with 100 mg/L ox-LDL significantly decreased cell viability and cholesterol efflux-related protein expressions, increased lipid uptake, accumulation and lipid uptake-related protein expressions, and significantly promoted inflammation and ROS level, as well as the expressions of myeloperoxidase (MPO), NADPH oxidase 2 (NOX2) in THP-1 macrophages (all P＜0.05). After pretreatment with theaflavin, cell viability was increased, intracellular lipid uptake, accumulation and lipid uptake-related protein expressions were significantly reduced, cholesterol efflux-related protein expressions were significantly increased, the expression and release of IL-6, IL-1β and TNF-α were significantly decreased, ROS level was significantly decreased, and the expression of MPO and NOX2 were decreased (all P＜0.05). Pretreatment with theaflavin effectively alleviated intracellular oxidative stress by altering the expression of NRF2, heme oxygenase-1 (HO-1) and Kelch-like ECH-associated protein 1 (KEAP1) in NRF2 signaling pathway, and enhanced the translocation of NRF2 into the nucleus. After pretreatment with ML385, the expression levels of NRF2, HO-1, KEAP1 and CD36 were significantly decreased. ConclusionTheaflavin can significantly inhibit ox-LDL-induced foam cell formation, inflammation, and oxidative stress through NRF2/HO-1 signaling pathway in THP-1 macrophages.