We study symmetry breaking of solitons in the framework of a nonlinear fractional Schrödinger equation (NLFSE), characterized by its Lévy index, with cubic nonlinearity and a symmetric double-well potential. Asymmetric, symmetric, and antisymmetric soliton solutions are found, with stable asymmetric soliton solutions emerging from unstable symmetric and antisymmetric ones by way of symmetry-breaking bifurcations. Two different bifurcation scenarios are possible. First, symmetric soliton solutions undergo a symmetry-breaking bifurcation of the pitchfork type, which gives rise to a branch of asymmetric solitons, under the action of the self-focusing nonlinearity.Second, a family of asymmetric solutions branches off from antisymmetric states in the case of selfdefocusing nonlinearity through a bifurcation of an inverted-pitchfork type. Systematic numerical analysis demonstrates that increase of the Lévy index leads to shrinkage or expansion of the symmetry-breaking region, depending on parameters of the double-well potential. Stability of the soliton solutions is explored following the variation of the Lévy index, and the results are confirmed by direct numerical simulations. * Recently, the investigation of SSB of solitons has been expanded into non-Hermitian optical systems, which are modeled by NLSEs with parity-time (PT ) symmetric complex potentials [32, 33], where families of stable asymmetric one-dimensional (1D) solitons induced by the SSB have been found for specially designed complex potentials [34-37], as well as under the action of 2D potentials [38]. An interesting generalization of the Schrödinger equation, corresponding to fractionaldimensional Hamiltonians, was proposed in Refs. [39]-[41]. It has been introduced, in the context of the quantum theory, via Feynman path integrals over Lévy-flight trajectories, which leads to the fractional Schrödinger equation (FSE). Although implications of such models are still a matter of debate [42, 43], some experimental schemes have been proposed for emulating them in condensed-matter settings and optical cavities [44, 45]. A number of intriguing dynamical properties have been predicted in the framework of FSEs, including 1D zigzag propagation [46], diffraction-free beams [47-51], beam splitting [52], periodic oscillations of Gaussian beams [53], beam-propagation management [54], optical Bloch oscillations and Zener tunneling [55], resonant mode conversion and Rabi oscillations [56], localization and Anderson delocalization [57], and SSB of PT -symmetric modes in a linear FSE [58].Naturally, FSEs may be applied to nonlinear-optical settings [59,60]. Recent works demonstrate that a variety of fractional optical solitons can be produced by nonlinear FSEs