Broad-spectrum resistance (BSR) reduces pathogen-related yield losses in crops such as rice (Oryza sativa). To achieve BSR, traditional breeding has focused on the time-intensive process of incorporating resistance (R) genes into elite germplasm. Now, CRISPR/Cas9-mediated genome editing makes it possible to modify susceptibility (S) genes to rapidly achieve BSR in rice. However, identifying S genes remains challenging. Here, we analyzed transcriptome data and determined that OsJAC1, encoding a mannose-binding jacalin-related lectin, is significantly induced upon infection by the rice blast fungus Magnaporthe oryzae. To explore the role of OsJAC1 in BSR in rice, we generated OsJAC1 overexpression and knockout mutant lines in the rice ZH11 (Zhonghua 11) background and performed pathogen inoculation assays, revealing that OsJAC1 negatively regulates resistance against M. oryzae and the bacterial blight pathogen Xanthomonas oryzae pv. oryzae. Further evidence of defense responses, such as a reactive oxygen species burst, defense-related gene expression, and MAPK phosphorylation, also supports the role of OsJAC1 as a negative regulator of plant immunity. To further validate the function of OsJAC1, we knocked out OsJAC1 in the Nipponbare (NPB) background. The resulting NPB-osjac1^cas9 plants showed enhanced defense responses and resistance against M. oryzae. Notably, the osjac1 mutants did not compromise agronomic traits in the ZH11 or NPB background. Hence, OsJAC1 could be regarded as an S gene and could serve as a potential target in rice breeding programs, providing valuable insights for crop improvement.