This work investigates mechanical reinforcement in styrene-butadiene rubber (SBR) composites containing sulfur-functional, organosilane-modified magadiite, a layered silicate (Na2Si1402g'9H20). Use of magadiite (MGD) enables us to vary the filler shape, surface area loading (filler area per mass of SBR), and silane graft density in MGDISBR composites. The goal is to rationalize the composites crosslink density and mechanical properties in terms of these variables to help us understand reinforcement mechanisms and energy dissipation in platelet-filled elastomers. Because Si69 does not graft onto as-prepared Na-MGD, we first treat MGD with cetyl trimethylammonium bromide (CTAB), resulting in CTA-MGD with expanded interlayer spacing. The interlayer surfaces of CTA-MGD platelets are silylated with either triethoxysilylpropyl-tetrasulfide (TESPT or Si69) or 3-mercaptopropyltriethoxysilane (MPTES). Surface-modified MGD materials (CTA-MGD, Si69-MGD, and MPTES-MGD) are mixed with SBR and cured to prepare elastomer composites. For composites prepared with CTA-MGD (with Si69 added during the mixing stage) or Si69-MGD with low Si69 graft density (I-Si69- MGD), XRD reveals significant expansion of the MGD interlayer spacing due to intercalation of elastomer. Composites based on CTA-MGD and I-Si69-MGD have crosslink density values comparable or greater than that of silica/SBR with equivalent filler loading. CTA-MGD/SBR and I-Si69-MGD/SBR composites manifest improved mechanical properties compared to silicalSBR. Overall, this work shows that sulfur-functionalized MGD has promise as an active filler for elastomer composites. Use of platelet shape and sulfur-functional silane graft density as formulation variables may give us additional options for tailoring mechanical reinforcement and energy dissipation in elastomer composites.