Background: Lung cancer is the second most predominant reason for cancer deaths globally. It is estimated to be approximately 30% among the cancer deaths. Daidzein (DAZ) is a polyphenolic compound present commonly in soy-based plants and is proven to have various therapeutic properties. Objectives: In this study, we aimed to understand the immunomodulatory activity of DAZ in the mice model with benzo(a)pyrene (B(a)P)-induced lung carcinoma. Materials and Methods: The mice were divided into five groups: Group I served was the control group; Group II animals were challenged with B(a)P; Group III animals were treated with DAZ before challenge with B(a)P; Group IV animals were treated with DAZ after challenging the animals with B(a)P; and Group V animals were treated with DAZ alone till the end of the experimental period. Tumor incidence was calculated, and the following parameters were analyzed: Body weight, lung weight, total number of tumors, percentage of inhibition, immunoglobulin (Ig) levels (immunoglobulin G, immunoglobulin A, and immunoglobulin M), key marker enzymes, and proinflammatory cytokines in both treated and normal mice. The lung tissues were analyzed through the histopathological analysis. Results: According to our results, all the markers that favor the growth of cancer were increased in the lung cancer group. After the administration of DAZ, all the markers returned to their original levels. Conclusion: In conclusion, DAZ protected the cells against the B(a)P-induced inflammatory responses in lung cancer.