This paper discusses the results of tensile tests performed on smooth S235 steel samples stored at ambient temperature and exposed to fire temperatures (700°C, 900°C, and 1100°C for 40 and 80 minutes). Acoustic emission (AE) measurements were taken during loading, along with force and deformation. The recorded AE events were analyzed individually and classified using a k-means algorithm. Analysis of the obtained results allowed the identification of acoustic emission signals characterizing various processes occurring during the operation of the steel material under load. The tests revealed significant differences compared to the material stored at ambient temperature and exposed to temperatures corresponding to fire conditions. A numerical model was developed based on the laboratory test results. The microstructure of the tested materials was also observed. The analyses conducted allowed for a reliable assessment of the feasibility of using the acoustic emission method for load monitoring and post-fire assessment of the condition of S235 steel. It was found that the acoustic emission method, due to differences at low loads, can be an effective technique for diagnosing the condition of steel after exposure to fire temperatures.