Polar codes are the first binary linear block codes provably achieving the symmetric capacity of arbitrary binary-input discrete memoryless channels. However, in their original design, their block length is limited to integer powers of two, a constraint that may be relaxed by puncturing. In this work, a novel construction of rateless codes based on punctured polar codes is presented, that is, codes that offer flexible rates via length adaption for a fixed dimension. While the approach presented relies on puncturing, it may be based on arbitrary puncturing methods. The rateless codes obtained work with standard polar code encoders and decoders, and allow for ad-hoc switching of the code rate without additional overhead.