WRKY proteins are a superfamily of plant transcriptional factors with potential regulatory roles pertaining to a variety of biotic and abiotic stress responses. In this study, we investigated the expression profiles of OsWRKY23 under different developmental stages, pathogen infection, continuous-dark and hormone treatments. Under normal growth conditions, OsWRKY23 expressed exclusively in roots and senescing leaves. Under biotic and abiotic stresses treatments, OsWRKY23 was markedly induced by continuous-dark-induced leaf senescence and infection by rice pathogen Pyricularia oryzae Cav as well as salicylic acid (SA). Further analysis of 35S:OsWRK23 Arabidopsis plants showed that over-expression of OsWRKY23 resulted in enhanced expression of the pathogenesis-related (PR) genes and increased resistance to the bacterial pathogen Pseudomanas syringae. Furthermore, over-expression of OsWRKY23 accelerated leaf senescence in darkness. The senescence-associated marker genes SAG12 and SEN1 were altered in darkness in 35S:WRKY23 Arabidopsis plants. In conclusion, these results suggest that OsWRKY23 is a novel modulator of pathogen responses as well as dark-induced leaf senescence.