In tropical and subtropical areas, peanuts are a very important legume crop that is widely cultivated for food and cooking oil. They are, however, extremely susceptible to a wide range of phytopathogens, particularly soil-borne fungi, which result in low yields and poor seed quality. This study aimed to express three Trichoderma asperellum SH16 42 kDa chitinase-encoding genes in peanut to improve their resistance to some soil-borne fungi. Chi42 is a synthetic, intronless, wild-type gene, whereas syncodChi42-1 and syncodChi42-2 are peanut codon-optimized synthetic genes. These genes were introduced into a local peanut strain through Agrobacterium-mediated transformation. Analysis of the transgenic peanut lines showed that chitinase-specific activities from the syncodChi42-1 and syncodChi42-2 genes were approximately 1.2 and 1.4 times higher than those of the wild-type gene, respectively. The engineered peanut plants also exhibited antifungal activity against the soil-borne pathogenic fungus Sclerotium rolfsii. The transgenic peanut lines transformed with the two synthetic genes have stronger antifungal activities than those transformed with the wild-type version, suggesting that they could be used as novel peanut lines to combat phytopathogenic fungi.