<p>High-performance biosensing is an important guarantee for the development of biology and biomedicine, especially the emergence of localized surface plasmon resonance (LSPR) sensing has brought more possibilities for biosensing. However, traditional LSPR terahertz sensors with low sensitivity and low Q-factor often do not provide surprising sample detection results, and in particular, dual detection with gases and liquids is extremely challenging. Here, we report the realization of highly sensitive terahertz biosensing of gases, liquids and biological cells via electromagnetic coupling between topological states. Two sharp dips are exhibited in the transmission spectrum of this topological state electromagnetic coupling (TSEC) biosensor, originating from the coupling between topological edge states (TES) in the waveguide and TES and topological corner states (TCS) in the cavity (i.e., TES-TES and TES-TCS couplings), respectively. The sensitivity and Q factors for TES-TES coupling (TES-TCS coupling) reach 2.16 (2.30) THz/RIU and 2.15 × 10⁶ (0.5 × 10⁶), respectively. Using the high sensitivity of the TSEC biosensor, we have achieved highly sensitive detection of gas concentrations and identification of different gas types, which shows great potential for applications in energy extraction and storage. Furthermore, through partially filled dielectrics, TSEC biosensors are capable of directly detecting biomolecules and live bacteria in solution, highlighting their powerful bio-detection capabilities. This use of electromagnetic coupling between topological states for ultra-high sensitivity and Q-factor terahertz biosensing provides an approach to multiple sensing of terahertz band excitations in gases, liquids, biomolecules, or biocells, as well as insight into environmental monitoring, gas exploitation, and biopharmaceuticals.</p>