Various types of magnetism can appear in emerging quantum materials such as van der Waals layered ones. Here, we report the successful doping of manganese atoms into a post-transition metal dichalcogenide semiconductor: SnSe$_2$. We synthesized a single crystal Sn$_{1-x}$Mn$_x$Se$_{2}$ with $textit{x}$ = 0.04 by the chemical vapor transport (CVT) method and characterized it by x-ray diffraction (XRD) and energy-dispersive x-ray spectroscopy (EDS). The magnetic properties indicated a competition between coexisting ferromagnetic and antiferromagnetic interactions, from the temperature dependence of the magnetization, together with magnetic hysteresis loops. This means that magnetic clusters having ferromagnetic interaction within a cluster form and the short-range antiferromagnetic interaction works between the clusters; a spin-glass state appears below ~ 60 K. Furthermore, we confirmed by $textit{ab initio}$ calculations that the ferromagnetic interaction comes from the 3$textit{d}$ electrons of the manganese dopant. Our results offer a new material platform to understand and utilize the magnetism in the van der Waals layered materials.
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