Background Step ascent and descent can perturb stability increasing the incidence of falls, especially in older individuals with functional limitations and intellectual disabilities, such as those with Down syndrome (DS). The aim of this study was to investigate the biomechanics and motor coordination of step ascent and descent in adults with DS and compare them with a group of healthy individuals, considering movement kinematics and kinetics. Method Fourteen adults with DS and 12 similarly aged adults without DS who were free of known motor problems were quantitatively assessed during ascending and descending a step using an optoelectronic system (BTS SMART‐D), force platforms and video recording. Kinematic and kinetic parameters were identified and calculated for each study participant and comparisons were made between the DS and a control group (CG). Results Despite similar age ranges, subjects in the DS group performed the step ascent and descent movements slower, with longer duration and with a more accentuated range of motion of the trunk and of the ankle joint than those in the CG. Additionally, the double stance phase on the step was substantially longer in the DS group when represented as a percentage of the entire stepping sequence (ascent, double stance on the step and descent). In terms of kinetics, ground force platform data revealed that the DS subjects showed higher instability in the medio‐lateral direction during double support phase than similarly aged CG subjects and cannot be attributed to age‐associated changes in stability. Conclusions These findings help to elucidate the complex biomechanical strategy of people with DS during a step ascent and descent movement task and may have a major role in the multidimensional evaluation and tailored management for them.