  We report the discovery of a temperature induced phase transition between the
\alpha and \beta structures of antimonene. When antimony is deposited at room
temperature on bismuth selenide, it forms domains of \alpha-antimonene having
different orientations with respect to the substrate. During a mild annealing,
the \beta phase grows and prevails over the \alpha phase, eventually forming a
single domain that perfectly matches the surface lattice structure of bismuth
selenide. First principles thermodynamics calculations of this van der Waals
heterostructure explain the different temperature-dependent stability of the
two phases and reveal a minimum energy transition path. Although the formation
energies of free-standing \alpha- and \beta-antimonene only slightly differ,
the \beta phase is ultimately favoured in the annealed heterostructure due to
an increased interaction with the substrate mediated by the perfect lattice
match.
