The present invention is to provide a method for growing a group III nitride crystal that has a large size and has a small number of pits formed in the main surface of the crystal by using a plurality of tile substrates. A method for growing a group III nitride crystal includes a step of preparing a plurality of tile substrates 10 including main surfaces 10m having a shape of a triangle or a convex quadrangle that allows two-dimensional close packing of the plurality of tile substrates; a step of arranging the plurality of tile substrates 10 so as to be two-dimensionally closely packed such that, at any point across which vertexes of the plurality of tile substrates 10 oppose one another, 3 or less of the vertexes oppose one another; and a step of growing a group III nitride crystal 20 on the main surfaces 10m of the plurality of tile substrates arranged.