The Cascadia region, particularly the Cascadia subduction zone, is capable of generating destructive megathrust earthquakes, making a comprehensive seismicity catalog essential for understanding its regional tectonics and assessing seismic hazard. Here, we present an offshore Cascadia earthquake catalog derived from four years of amphibious data from the Cascadia Initiative array and complementary seismic deployments. Continuous data were analyzed using deep-learning pickers, OBSTransformer and EqTransformer, respectively. Earthquake association and initial locations were obtained using three independent methods (REAL, Gamma, and PyOcto); only events identified by all three were retained. Locations were further refined using absolute and double-difference relocation techniques. To improve catalog completeness, especially in regions of low seismicity, we applied the match-filter method. Our catalog contains ~27,000 earthquakes with magnitudes ranging from −2.7 to 5.1. Approximately 22,500 events align with mapped structures of the Blanco Transform Fault System (BTFS), revealing fine-scale fault segmentation and several persistent seismicity gaps extending to Moho depths. Analysis of teleseismic stress perturbations indicates delayed dynamic triggering along multiple BTFS segments, with the Surveyor Depression exhibiting heightened sensitivity. The catalog further includes ~3,500 events in the Gorda Plate, outlining a previously unrecognized fault near the Mendocino Triple Junction, and ~700 events within the Juan de Fuca Plate, including a dense cluster consistent with active serpentinization inferred from Vp/Vs and reflection data. Along the Cascadia margin, ~600 events illuminate seamount–margin interaction near 42°N and newly identified intraslab seismicity offshore Oregon. This catalog provides new constraints on offshore Cascadia tectonics and seismic hazard.