A seat assembly (10) for use in automotive vehicles includes a seat frame (18) and an armrest frame (20) that is pivotally supported on the seat frame (18) for pivotal movement between an armrest position and an upright position. An inertia lock member (26) that is pivotally supported on the armrest frame (20) pivots into engagement with a stop member (24) that is supported on the seat frame (18) when the armrest (16) is in the upright position and subjected to a predetermined inertia force (31) under certain vehicle deceleration conditions. The inertia lock member (26) is operative to maintain the armrest (16) in the upright position and allows a seat occupant to move the armrest (16) into an armrest support position under normal use conditions.