Organic materials are gaining increasing interest as electrode materials for Lithium Ion Batteries (LIBs) due to their molecular diversity, high capacity, low cost, environmental friendliness and durability. However, most organic electrode materials suffer from poor electronic conductivity, limited active sites and high solubility in organic electrolytes, which impede their practical use in LIB applications. To avoid these drawbacks, an insoluble redox-active porous polymer (RAPP) based on triazine and pyromellitic diimide has been prepared by a simple one-pot solvothermal polymerization method and employed as a cathode material for LiBs. The poly(imide-triazine), denoted as PIT, shows dual porosity combining micro and mesopores with a high specific surface area of about 674 m$^2$ g$^{-1}$ which facilitates electrolyte penetration and ionic transportation and provides interfacial Li storage allowing an additional capacity. Moreover, the dual-redox active sites of C[double bond, length as m-dash]N and C[double bond, length as m-dash]O groups generate more capacity. Remarkably, the LiBs assembled with the synthesized PIT cathode show continuous activation with a high discharge capacity of 233 mA h g$^{-1}$ after 300 cycles at 2 A g$^{-1}$. After an activation process, the PIT material delivers a very high discharge capacity (422 mA h g$^{-1}$ at 0.5 A g$^{-1}$), excellent rate capability (268 mA h g$^{-1}$ at 10 A g$^{-1}$) and extraordinary ultralong cycling stability keeping 65% of discharge capacity after 4000 cycles at 6 A g$^{-1}$. These unprecedented performances outperform most organic/polymeric cathode materials, making PIT a highly promising cathode material for LiBs for future electrochemical energy-storage applications.