Obesity, a lifestyle disease resulting from excessive caloric intake and insufficient physical activity, results in a state of chronic inflammation. A food ingredient that suppresses chronic inflammation could help prevent associated diseases. Sweet basil (Ocimum basilicum L.) is a herb from the Lamiaceae family with some reported anti-inflammatory effects. Via this in vitro study, we aimed to investigate whether sweet basil exerts anti-inflammatory effects in obese patients. Fresh sweet basil leaves were freeze-dried and powered. After that, this was extracted with 80% methanol. After 3T3-L1 adipocytes were cultured with sweet basil extracts at final concentrations of either 5 or 25 μg/mL for 24h, RAW264.7 macrophages were seeded onto this adipocytes and co-cultured for 12h. We determined the effects of sweet basil extracts on inflammatory cytokine expression by real-time PCR or western blotting. Sweet basil extracts reduced the expression of inflammatory cytokine mRNA induced by co-culture, including that of IL-6 (Il6), IL-1β (Il1b), TNF-α (Tnf), and CCL2 (Ccl2). In addition, sweet basil extracts suppressed the mRNA expression of NF-κB (Nfκb1), a transcription factor of inflammatory cytokines. In an investigation of costimulatory CD137 (Tnfrsf9)/CD137L inflammatory signaling, a member of the TNF super-family, sweet basil extracts inhibited Tnfrsf9 expression induced by the co-culture. Therefore, the results of this study indicated that sweet basil extracts have an anti-inflammatory effect against adipocyte-induced inflammation, possibly through suppression of Tnfrsf9 expression.