This paper presents a comprehensive study on the active stiffness modulation of flexible structures using Shape Memory Alloy (SMA)‐elastomer composites. We developed a robust fabrication process for creating NiTi wire‐reinforced Polydimethylsiloxane (PDMS) composites. The effective Young's modulus of the composite was characterized experimentally over a temperature range from −5 to 90℃, revealing a threefold stiffness increase (from ~15 to ~45 MPa) when the NiTi wires are heated above their austenite finish temperature. Both theoretical analysis based on the rule of mixtures and finite element simulations showed excellent agreement with the experimental data, which paves the way for design and implementation of active stiffness modulation of the composite's macroscopic stiffness by direct Joule heating. This work establishes a foundational framework for developing variable‐stiffness flexible structures with potential applications in soft robotics, morphing skins, and aerospace deployables.