A lumbar support device that is adjusted to a user's neutral postural position when the user is standing, and that is subsequently used to reinforce the user's neutral postural position when the user is seated. The device includes a rigid shell and an adjustable lumbar support surface. The lumbar support surface is a plurality of inelastic straps of adjustable length disposed horizontally between the sides of the rigid shell. The user adapts the straps to conform to the user's back when in a standing neutral position. When seated, the straps maintain their adjusted lengths, so as to reinforce the user's neutral position. The device includes a covering and a padding between the covering and the straps. The lumbar support device features downwardly depending legs at each side so as to support the device when placed in a chair and position the surface in the lumbar region above the chair. The gap between the legs and below the back of the shell provides a cutout to accommodate protrusion of the user's buttocks, allowing the user to adopt normal buttock positioning on the seat of the chair.