  We demonstrate phase control of magnons in the van der Waals antiferromagnet NiPS$_3$ using optical excitation by polarized light. The sign of the coherent precession of spin amplitude changes upon (1) reversing the helicity of a circularly polarized pump beam, or (2) rotating the polarization of a linearly polarized pump by $\pi/2$. Because these two excitation pathways have comparable generation efficiency, the phase of spin precession can be continuously tuned from 0 to $2\pi$ by controlling the polarization state of the pump pulse. The ability to excite magnons with a desired phase has potential applications in the design of a spin-wave phased array and ultrafast spin information processing. 
Phase control of magnons in the van der Waals antiferromagnet NiPS$_3$