Polymer-infiltrated nanoparticle films (PINFs) are a new class of polymer nanocomposites that overcome many of the challenges in manufacturing highly loaded nanocomposite films (PNCFs). PINFs have a wide range of applications as functional coatings and membranes and have excellent mechanical properties, including high stiffness, hardness, scratch and wear resistance. Fracture toughness is another critical mechanical property that plays an important role in the durability and reliability of PINFs. Establishing a fundamental understanding of the fracture behavior of PINFs is crucial to the design of damage tolerant PINFs.In this work, the effect of particle shape, confinement, and polymer-nanoparticle (NP) interactions on the fracture behavior of PINFs is investigated. The results show that well-aligned, high aspect ratio (AR) nanoplatelet films have significantly higher fracture toughness and larger enhancement in modulus, hardness and scratch resistance upon polymer infiltration than low AR NP films. Using a thin-film fracture testing method based on the double cantilever beam specimen, the role of confinement on fracture toughness is investigated by tuning the polymer molecular weight (MW) and NP size. The effect of polymer-NP interaction strength is also investigated by varying the type of polymer and changing the surface chemistry of NPs. The results show that the polymers can significantly toughen NP films through a confinement-induced molecular bridging mechanism when they are under extreme confinement and completely lose interchain entanglement. This mechanism is controlled by polymer MW, pore size and polymer-NP interaction strength and ultimately limited by the strength of polymer backbone. As the degree of confinement decreases, the bridging mechanism becomes less pronounced. In this case, entanglement-based mechanism becomes dominant and the fracture toughness of PINFs is dependent on the fracture properties of polymers. Finally, it is shown that the fracture toughness of PINFs can be further enhanced using mesoporous NPs due to the increase of polymer-NP interaction area and the interlocking between the polymer and NPs via mesopores. Thus, this work provides important guidelines for the design of mechanically robust PINFs as well as other highly loaded PNCFs.