Balance during walking is of high importance to prosthesis users and may affect walking during baseline observation and evaluation. The aim of this study was to determine whether changes in walking balance occurred during an adaptation period following the fitting of a new prosthetic component. Margin of stability in the medial-lateral direction (MOSML) and an anterior instability margin (AIM) were used to quantify the dynamic balance of 21 unilateral transtibial amputees during overground walking. Participants trialled two prosthetic feet presenting contrasting movement/balance constraints; a Higher Activity foot similar to that of their own prosthesis, and a Lower Activity foot. Participants were assessed before (Visit 1) and after (Visit 2) a 3-week adaptation period on each foot. With the Higher Activity component, MOSML decreased on the prosthetic side, and increased on the sound side from Visit 1 to Visit 2, eliminating a significant inter-limb difference apparent at Visit 1 (Visit 1–sound = 0.062 m, prosthetic = 0.075 m, p = 0.018; Visit 2–sound = 0.066 m, prosthetic = 0.074 m, p = 0.084). No such change was seen with the Lower Activity foot (Visit 1–sound = 0.064 m, prosthetic = 0.077 m, p = 0.007; Visit 2–sound = 0.063 m, prosthetic = 0.080 m, p < 0.001). Significant changes in AIM were observed at Visit 2 (Visit 1: −0.16 (0.08) m, Visit 2: −0.17 (0.08) m; F = 23.396, p < 0.01). These findings suggest that changes in balance during walking can occur following the initial receipt of a device regardless of whether the component is of the same functional category as the one an individual is accustomed to using.