In V3Si, a first-order structural phase transition from hexagonal to square flux-line lattice occurs at approximately 1 T with H to the a axis. In this paper, we demonstrate the reentrant structural transition in the flux-line lattice, which reverts to hexagonal symmetry as the magnetic field approached Hc2(T). This behavior is described very well by a nonlocal London theory with thermal fluctuations. The phase diagram of the flux lattice topology is mapped out for this geometry. © 2005 The American Physical Society.