<p>Cooling a material into a ferromagnetic phase can produce arbitrary metastable patterns of magnetic domains rather than a spatially uniform magnetic state. Control over the formation of these patterns could provide non-chemical methods of creating spintronic devices. Here we demonstrate high-efficiency optical training of magnetic domain formation in the two-dimensional van der Waals magnet Fe₃GeTe₂ during zero-field cooling. At ultralow power densities of around 20 µW µm⁻², electrons excited by linearly polarized photons catalyse the formation of larger domains for both spin orientations. Furthermore, circularly polarized photons of the same low power density produce a single domain with its magnetization orientation determined by the optical helicity. We propose that the emergence of this single domain is caused by the optically injected spin-polarized electrons acting as initial magnetic seeds that guide different regions of the sample into the same spin orientation. Our work presents an unconventional route to tailoring spin textures in two-dimensional materials.</p>