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Soft, stretchable, sewable, and strain‐sensitive optical fibers are reported. Intrinsically stretchable optical waveguides capable of >100% strain are formed by coating a commercially available elastic urethane fiber (“Stretch Magic”) with a silicone cladding having a lower refractive index. These 1–2 mm diameter fibers can be attached to textiles using a sewing machine. Because the attaching threads create microbends in the fiber, the sewn fibers are prestrained for monotonic intensity‐versus‐strain behavior. Installed in a stretchable piece of athletic tape, they detect strains originating from changing muscle shapes during weight‐bearing activity.