Conventional directional antennas in the marine buoy applications exhibit signal blind zones that degrade communication link quality. In this paper, we propose a compact dual-band isotropic antenna employing an orthogonal dual-arm structure. First, a novel tri-fold current distribution model with equal magnitude is presented, and the underlying mechanism for achieving three-dimensional (3D) quasi-isotropic radiation is accurately analyzed. Subsequently, leveraging the principle that orthogonal current distribution minimizes mutual coupling between two ports, a spatially perpendicular dual-arm configuration is further constructed and simulated to realize dual-band 3D quasi-isotropic radiation characteristics. Simulation results verify that the proposed antenna achieves 3D omnidirectional radiation in both operational bands of 1.57–1.69 GHz and 2.06–2.25 GHz, where the realized gain variation in 3D sphere space is lower than 3.89 dB and 2.1 dB, respectively. Besides, the port isolation is higher than 50 dB in the simulated results. Due to the merits of compact structure, quasi-isotropic radiation and dual-band properties, the proposed antenna shows promising potential for marine buoy systems.