The design of the Secondary Mirror Subassembly (SMSA) in the 2.4m NASA Optical Telescope is presented. The secondary mirror is a circular disc provided with a shoulder. Three clamps located 120 apart support the mirror shoulder. On one side of the mirror shoulder, a coated graphite-epoxy ring connects the three clamps. Three axial flexures with a low radial compliance extend out from a support platform constructed of coated graphite-epoxy box struts and invar corners, and provide a nominal in-plane zero coefficient of thermal expansion (CTE). In each invar corner piece, a ball joint provides a connection to a six-degree-of-freedom adjustment system that can move the secondary mirror with a high degree of precision in defocus, decenter, and tilt. Thermal controls that maintain structural-element temperature variations within specified limits insure that the SMSA exceeds stability and figure control requirements on the secondary mirror.