Vitreoretinal surgery requires exceptional precision, stability, and dexterity. The issues related to retinal surgery include, (a) potential sclera stress and intraocular lens collision induced by the use of conventional rigid straight tools, and (b) the small tool-to-tissue interaction force that is normally imperceptible to surgeons. Recently, concentric tube-based robots (CTRs) have been adopted to solve the aforementioned issues in vitreoretinal surgery. However, the tip force sensing capability in CTRs owing to their dimensional constraints for sensor embedding is one of the challenging issues that CTRs face. This work introduces a novel slot-based force sensing module to be used in a sub-millinewton force sensing concentric tube (CT)-based eye surgery robot. The module is composed of half-cut slots and fiber Bragg grating (FBG) sensors. The force sensing module was modeled analytically using the composite beam theory to analyze its characteristics. To verify the analytical model, finite element analysis (FEA) was carried out in a simulation environment, and experiments using a prototype were performed to obtain the force–strain relationship. The FEA and experimental results show that the composite beam model is highly compatible with the force sensing module and that the module can detect transverse forces with a resolution of 0.25 mN. It was also verified that the slot-based force sensing module can be integrated into the inner tube of a CT-based eye surgery robot with a diameter of 0.43 mm.