Thin and flexible photonic sensor foils are proposed, fabricated, and tested as a promising alternative for monitoring composite structures. Sensor foils are implemented using two different optical polymers and as such optimized for multi‐axial sensing and embedding within composite materials, respectively. It is first shown that those sensor foils allow multi‐axial strain sensing by multiplexing a multitude of Bragg grating sensors in a rosette configuration. Secondly, those sensors can be realized in very thin foils (down to 50 µm) making them suitable for embedding in composite materials during their production. This is proven by visually inspecting and by testing the functionality of the embedded sensors. Finally, owing to their low Young's modulus and flexibility, polymer sensor foils can be bent to small curvature radii and withstand large elongations. Herein, the sensors are bent down to a radius of 11 mm, and elongated by 1.4% without losing functionality. Thin and flexible photonic sensor foils allow multi‐axial strain sensing of composites by multiplexing a multitude of Bragg grating sensors. Those foils can be made very thin (down to 50 μm) making them suitable for embedding. Owing to their low Young's modulus and flexibility, polymer sensor foils can be bent to small curvature radii and withstand large elongations.