Hypertensive heart failure highlights an urgent need for effective therapeutic strategies. Protein kinases regulate multiple pathways in cardiac pathophysiology and may provide promising therapeutic targets. Here, we identified a Cyclin-dependent kinase, CDK9, promoting inflammation and cardiac remodeling in terminally differentiated cardiomyocytes. Firstly, kinase enrichment analysis and experimental evidence revealed CDK9 phosphorylation at Thr-186 in both human and mouse hypertrophic heart tissues. CDK9 loss of function via T186A mutation in cardiomyocytes attenuated Ang II-induced heart remodeling and NF-κB-mediated inflammation, whereas CDK9 overactivation by T186E mutation induces. This regulatory function of CDK9 in cardiac remodeling is cell cycle-independent. Further studies demonstrate that the kinase domain of CDK9 directly binds to NF-κB P65 protein, which leads to the CDK9/P65 complex nuclear translocation, P65 phosphorylation, and transcription of inflammatory and hypertrophic genes in cardiomyocytes. This process requires CDK9 Thr-186 phosphorylation and Cyclin T1 presence, but is independent on IKKβ and CDK9-RNAPII pathways. Pharmacological inhibition of CDK9 phosphorylation significantly attenuated Ang II-induced cardiac inflammation, remodeling, and dysfunction in mice. Collectively, Ang II-activated CDK9 directly binds to and phosphorylates P65 to drive cardiac inflammation and remodeling. This study identifies CDK9 as a potential target in heart failure therapeutics