A new type of diffractive X-ray optical elements is reported, which have been used as beam-shaping condenser lenses in full-field transmission X-ray microscopes. These devices produce a square-shaped flat-top illumination on the sample matched to the field of view. The size of the illumination can easily be designed depending on the geometry and requirements of the specific experimental station. Gold and silicon beam-shapers have been fabricated and tested in full-field microscopes in the hard and soft X-ray regimes, respectively.