Appropriate sports bras are crucial to limit potential breast pain and ptosis. In an attempt to optimize breast support during exercise, manufacturers now produce activity level-specific sports bras. However, until breast movement across activity levels is understood, the criterion for such apparel is unknown. Therefore, the aim of this study was to quantify multi-planar breast displacement across treadmill activity levels and breast support conditions. Twenty-one D cup participants had markers attached to their nipples and trunk to calculate relative 3D breast displacement. Supported and unsupported mediolateral, anterioposterior, vertical, and resultant breast displacement was assessed during treadmill walking up to maximum running. Unsupported resultant breast displacement increased from 4.2 ± 1.0 cm during walking to 15.2 ± 4.2 cm during running. There was no change in breast displacement amplitude or direction as running speed increased above 10 km · h-1, with vertical breast displacement then accounting for ∼50% of overall displacement. While breast support was effective in reducing the amplitude of breast displacement during walking and running, the direction was unaffected. In conclusion, instead of activity level-specific breast support, future research should identify multi-planar breast kinematics during various sporting modalities. This might lead to production of a sports-specific bra to reduce the negative consequences of breast movement.