Abstract Background Monitoring activity‐related energy expenditure ( EE ) is essential in the management of daily activity and the dietary programme in patients with type 2 diabetes (T2D) and must be estimated accurately. Accelerometry‐based equations have frequently used to estimate EE , although the validity of these methods has not been confirmed in patients with T2D. The present study aimed to test the validity of an accelerometry‐based method (Bouten's method) to assess EE during walking in patients with T2D. Methods The study included 20 patients with controlled T2D [mean (SD) duration: 10.6 (6.1) years; age: 57.5 (8.4) years; body mass index: 26.4 (2.6) kg m − ²]. All participants performed five 6‐min periods of walking at different speeds (0.5–1.5 m s −1 ) on a treadmill. Mechanical data were recorded using an inertial measurement unit placed on the lower back with gas exchange being simultaneously monitored. Results Values of EE during walking estimated by the accelerometer method did not differ significantly from those measured by indirect calorimetry. Bias and root mean square error were −1.17 and 2.93 kJ min −1 , respectively, on average across speeds. Conclusions Our results suggest that EE during walking may be accurately estimated in patients with diabetes mellitus using an accelerometer.