Of the nearly 3000 radio pulsars currently known, only about 300 are in binary systems, and only 5 of these consist of young pulsars with massive non-degenerate companions. We present the discovery and initial timing of the 6th such binary pulsar, PSR J2108+45, a 0.577-s radio pulsar in a ~270-day orbit around a companion of minimum mass ~11 solar masses, with the Canadian Hydrogen Intensity Mapping Experiment telescope. The system has a low eccentricity of ~0.09 and a spin-down luminosity of 1033 erg/s. Notably, the system undergoes periods of substantial eclipse, disappearing from the 400-800 MHz observing band for a large fraction of its ~270 day orbit, and significant dispersion measure and scattering variations, pointing to the possibility of a circumstellar disk or very dense stellar wind. Sub-arcsecond resolution imaging with the Karl G. Jansky Very Large Array unambiguously localized the system to an O/B/Be-type main sequence companion, located at a distance of ~3 kpc. With further multiwavelength followup, PSR J2108+45 promises to serve as another rare laboratory for the exploration of companion winds, circumstellar disks, and short-term evolution through extended-body orbital dynamics.