Light-matter interactions in conventional nanophotonic structures typically lack directionality. For example, differently from microwave antenna systems, most optical emitters (e.g., excited atoms/molecules and simple nanoantennas) exhibit quasi-isotropic dipolar radiation patterns with low directivity. Furthermore, surface waves supported by conventional material substrates do not usually have a preferential direction of propagation, and their wavefront tends to spread as it propagates along the surface, unless the surface or the excitation is properly engineered and structured. In this article, we theoretically demonstrate the possibility of realizing unidirectional and diffractionless surface plasmon polariton modes on a nonreciprocal platform, namely, a gyrotropic magnetized plasma. Based on a rigorous Green’s function approach, we provide a comprehensive and systematic analysis of all the available physical mechanisms that may bestow the system with directionality, both in the sense of one-way excitation of surface waves and in the sense of directive diffractionless propagation along the surface. The considered mechanisms include (i) the effect of strong and weak forms of nonreciprocity, (ii) the elliptic-like or hyperbolic-like topology of the modal dispersion surfaces, and (iii) the source polarization state, with the associated possibility of chiral surface-wave excitation governed by angular-momentum matching. We find that three-dimensional gyrotropic plasmonic platforms support a previously unnoticed wave-propagation regime that exhibit several of these physical mechanisms simultaneously, allowing us to theoretically demonstrate unidirectional surface plasmon polariton modes that propagate as a single ultranarrow diffractionless beam. We also assess the impact of dissipation and nonlocal effects. Our theoretical findings may enable a new generation of plasmonic structures and devices with highly directional response.