Hind-leg activity in dairy cows during milking is considered to be an indicator of a stressful situation or discomfort. Automatic detection of cow hind-leg activity during milking could be used to monitor deficiencies at the cow, milking machine, or human level. We assumed that the milking cluster follows cow hind-leg movements during milking. Therefore, the aim of our study was to estimate the ability of a 3D-accelerometer attached to the claw’s hook of the milking cluster to indirectly measure cow hind-leg activity during milking. The behavior of 45 dairy cows during 1 morning and 1 evening milking in a 2 × 3 auto tandem milking parlor was monitored by using direct visual observations and a 3D-accelerometer attached directly to the hook of each milking claw. We recorded when hind-leg foot-lifting or stepping (undifferentiated) and kicking behaviors occurred. Detected activities were obtained after applying a standard deviation filter (over a sliding window of 1 s) on the x (left/right lateral) acceleration axis only. We set the detection threshold at 0.13 g (i.e., 1.27 m s−2), based on the visual evaluation of 10 “reference” graphs (from 5 randomly chosen cows). Periods of activity were created for each observed or detected activity (and for consecutive activities that were less than 3 s apart) to take into account the milking cluster rocking motion resulting from a single movement. Matching observed and detected periods of activity were considered when they occurred simultaneously (with a 2 s delay acceptance in the visual observations). In total, 472 periods of hind-leg activity were observed. The detection of the cows’ hind-leg activity by using a 3D-accelerometer attached to the milking cluster was found to have 69% sensitivity, 99% specificity, 75% positive predictive value, 98% negative predictive value, and an overall accuracy of 97%. On average, the cows were observed being active 0.94 times per minute while being detected active 0.86 times per minute. The observed and detected periods of activity were found to be correlated (r = 0.657). The use of a 3D-accelerometer attached to the milking cluster seems therefore promising for reliably estimating cow hind-leg activity during milking. Nevertheless, further investigations remain necessary to clarify whether other factors can cause milking cluster movement variations regardless of cow hind-leg activity and to generalize our method to better identify deficiencies in the cow-machine-human interactions.