This paper investigated three-dimensional breast displacement under different breast support conditions and gait speeds for women with small breasts. The breast coordinates of fifteen female participants with small breasts (sizes ranging from A to C) were recorded during treadmill walking and running tests under two different sports bra conditions and a no bra condition. The results showed that although vertical breast displacement was always the greatest in each condition, mediolateral breast displacement was significantly greater than anteroposterior breast displacement. Mediolateral breast displacement was not effectively reduced in the two sports bra conditions compared to the no bra condition. It is recommended that sports bra designed for women with small breasts should aim to limit mediolateral breast displacement instead of anteroposterior breast displacement, on the premise of controlling vertical breast displacement. The findings also suggest that the effectiveness of sports bras at reducing side-to-side breast movement requires further optimisation.