This letter presents the first epsilon-near-zero (ENZ) substrate integrated waveguide (SIW) resonator that yields a more than 2× quality factor increases compared with conventional SIW resonators. The presented SIW resonator achieves this performance through a supercavity (SC) mode excited right next to an avoided mode crossing (AMC) where its effective permittivity approaches zero. A high-index resonator (HIR) is required for this implementation and is integrated inside the SIW resonator requiring no additional volume. The proposed ENZ SIW resonator demonstrates a measured unloaded quality factor of 490 (720 in simulations) at 2.06 GHz. The resonator is implemented in a conventional Roger TMM13i 0.075ʹʹ substrate. It occupies an area of only 0.035 λ2 , where λ=λ0/ε√r,λ0 is the free-space wavelength, and εr=12.2 in the dielectric permittivity of the SIW waveguide. The same structure demonstrates a measured unloaded quality factor of only 217 (350 in simulations) when excited with a conventional TM11 mode away from the ENZ mode region. The presented results demonstrate significant potential of this technology in advanced filter architectures.