This study examines the performance of the refined time-reversal method (RTRM), which recommends best reconstruction frequency as probing frequency and uses an extended mode of the reconstructed signal for computing the damage/similarity index for damage detection under a varying thermal environment. The experiments were performed on a 3 mm aluminum plate with a block mass-type damage at three excitation frequencies: the best reconstruction frequency, the sweet spot frequency at which only S0 mode is actuated, and an arbitrarily chosen frequency. Measurements were taken at two temperatures, 25∘ C and 50∘ C. The percent similarity between the reconstructed and original input signals showed a consistent decrease with the increase in damage height at the best reconstruction frequency. However, at the other excitation frequencies, the change in the similarity with the increase in damage height did not follow a consistent trend, which can lead to erroneous information about damage size. The percent similarity changes significantly with temperature rise at the sweet spot frequency and the arbitrarily chosen frequency. However, it showed minimal change with temperature rise at the best reconstruction frequency for undamaged and damaged states of different severity, making the RTRM suitable for baseline-free damage detection and sizing even under temperature variations. © 2023, The Author(s), under exclusive license to Springer Nature Switzerland AG.