Background Objective measurement tools may be of great value to provide early and reliable burn wound assessment. Thermal imaging is an easy, accessible and objective technique, which measures skin temperature as an indicator of tissue perfusion. These thermal images might be helpful in the assessment of burn wounds. However, before implementation of a novel measurement tool into clinical practice is considered, it is appropriate to test its clinimetric properties (i.e. reliability and validity). The objective of this study was to assess the reliability and validity of the recently introduced FLIR ONE thermal imager. Material and methods Two observers obtained thermal images of burn wounds in adult patients at day 1–3, 4–7 and 8–10 after burn. Subsequently, temperature differences between the burn wound and healthy skin (ΔT) were calculated on an iPad mini containing the FLIR Tools app. To assess reliability, ΔT values of both observers were compared by calculating the intraclass correlation coefficient (ICC) and measurement error parameters. To assess validity, the ΔT values of the first observer were compared to the registered healing time of the burn wounds, which was specified into three categories: (I) ≤14 days, (II) 15–21 days and (III) >21 days. The ability of the FLIR ONE to discriminate between healing ≤21 days and?>21 days was evaluated by means of a receiver operating characteristic curve and an optimal ΔT cut-off value. Results Reliability: ICCs were 0.99 for each time point, indicating excellent reliability up to 10 days after burn. The standard error of measurement varied between 0.17–0.22 °C. Validity: the area under the curve was calculated at 0.69 (95% CI 0.54–0.84). A cut-off value of −1.15 °C shows a moderate discrimination between burn wound healing ≤21 days and >21 days (46% sensitivity; 82% specificity). Conclusion Our results show that the FLIR ONE thermal imager is highly reliable, but the moderate validity calls for additional research. However, the FLIR ONE is pre-eminently feasible, allowing easy and fast measurements in clinical burn practice.