The hemibiotrophic bacterial pathogen Pseudomonas syringae (Pst) infects a range of plant species and causes enormous economic losses. Despite its agronomic significance, the molecular mechanisms underlying tomato–Pst interactions remain largely uncharacterized. To elucidate these mechanisms, we conducted a comprehensive transcriptomic analysis using infected tomato leaves inoculated with virulent strains Pst DC3000 at relatively early time points. RNA-sequencing of nine libraries identified stage-specific expression patterns, with DEG counts ranging from 484 to 1267 upregulated and from 560 to 844 downregulated genes. Enrichment analysis highlighted significant alterations in metabolic pathways, plant–pathogen interaction networks, and hormone signaling cascades, with marked transcriptional reprogramming observed between the pre- and post-infection stages. A longitudinal analysis of gene expression dynamics identified 15 consistently upregulated and 9 downregulated genes across all post-inoculation time points. Notably, in several candidate genes, a homologous gene of AtNSP2, SlNSP-Like was confirmed to be involved in disease resistance in tomato leaves. SlNSP-Like is localized in the cytoplasm and nucleus, and the transient overexpression of SlNSP-Like tomato plant exhibits significant resistance to Pst DC3000. This study provides valuable insights into the molecular dialogue between tomato and Pst, and the identified regulatory genes and pathways serve as promising targets for breeding disease-resistant tomato cultivars and developing management strategies against bacterial spot disease.