<p>Traditional cancer screening and therapy techniques involve time-consuming, costly, and painful surgeries, chemicals, and radiotherapy. Early cancer detection approaches that are sensitive, affordable, and rapid have sparked significant attention. Optical biosensors offer great sensitivity, label-free detection, and compact size. This work proposes a novel design and simulation of Theta Bar-Square Resonator-based Refractive Index Biosensor (TBSRRIB) for the recognition of basal cells (skin cancer) and HeLa cells (cervical cancer). The suggested biosensor is working on the surface plasmon resonance phenomena on a presented theta-shaped geometry design. The surface plasmon resonance (SPR) methodology using silver (Ag) as the resonator material in the TBSRRIB involves coating the biosensor with a thin layer of silver, which has excellent plasmonic properties which includes strong plasmonic resonance and minimal energy loss. When light hits the silver-coated surface, it stimulates surface plasmons, which will result in an evanescent waveform that penetrates a short distance across the medium of dielectric. As the target cancerous cells interact with the biosensor’s recognition components, the coefficient of refractive index within the metal-dielectric interface shifts, which leads to a variation in the transmittance rates and wavelength of the reflected light. An important parameter analysis has been done and the same preeminent outcomes have been shown with silver (Ag) material as a plasmonic coating material. The biosensor possesses a preeminent sensitivity of 1083.33 nm/RIU for HeLa cancerous cells, a preeminent quality factor value of 1177.09 nm/RIU, and a preeminent figure of merit value of 801.52 for the cancerous basal cells. The efficient detection limit value of 0.000413 has been obtained for skin cancer cells.</p>