Understanding entropic contributions to ordering transitions is essential for the design of self-assembling systems with tunable hierarchical structures. Herein, we report entropy-mediated precise interfacial organization of Janus nanoparticles in the flexible semiflexible block copolymers and the resulted optical properties of this heterogeneous material by combining coarse-grained molecular dynamics and a finite difference time domain technique. We find that the stiffness of the semiflexible block can regulate the off-center distribution of symmetric Janus nanoparticles with respect to phase interfaces, featured by a roughly 35% deviation from the interface to the utmost extent. Our simulations reveal how entropic and enthalpic effects in this multiphase media contribute to the self-assembled morphologies and, in particular, can lead to novel chain stiffness-induced entropy effects that can be harnessed to tailor the interfacial organization of Janus nanoparticles in the scaffold of block copolymers. Furthermore, the combination of techniques allows us to determine how changes of the interfacial nanostructures affect the optical properties of the nanocomposite. The findings enable the applications of polymer chain stiffness in precise control over the interfacial assembly of nanoparticles in heterogeneous materials and provide guidelines for facilitating the design of photonic crystals.