Bound state in the continuum (BIC) has been considered a promising strategy to reduce fabrication complexity and radiative loss. Despite the successful demonstration of many BIC-based configurations, their mode confinement is comparable to conventional dielectric waveguides. Even though plasmonics with strong light localization have been introduced into BICs, the intrinsic ohmic loss is still a facing challenge to be conquered. Herein, we propose a new approach, to our knowledge for the first time, that integrates subwavelength confinement and optical BIC in all dielectric slot waveguides. A nanogap with low refractive index is utilized to separate the top waveguide and high-index substrate, leading to strong mode localization and energy confinement (∼ λ2/30). Surprisingly, leveraging on the concept of optical BICs, the leakage channels of the waveguide bound mode can be readily engineered to cancel each other through destructive interference, which effectively suppresses the optical dissipation. Moreover, the subwavelength confinement is observed in ultrahigh-Q microring resonators, resulting in a giant enhancement of the Purcell factor (> 108). Our findings introduce a new concept into the BIC family and provide novel thoughts to design BIC photonics, which may show superiority in achieving low threshold lasers and highly sensitive sensors.