The article introduces caDNAno, an open-source software package designed to aid in the rapid prototyping of 3D DNA-origami shapes. DNA nanotechnology allows for the self-assembly of complex structures using base-pairing specificity. The authors previously adapted the 'scaffolded DNA origami' method to create 3D shapes formed by pleated layers of double helices constrained to a honeycomb lattice. However, the design process was cumbersome and time-consuming. CaDNAno simplifies this process by providing a graphical user interface that facilitates the design of DNA sequences for folding 3D honeycomb-pleated shapes. The software is demonstrated through the design, assembly, and analysis of a series of rectangular-block motifs, which were found to be well-behaved building blocks for future studies. The use of caDNAno significantly reduces the effort required for designing 3D DNA-origami structures, and the software is available at http://cadnano.org/ with example designs and video tutorials. The source code is released under the MIT license.The article introduces caDNAno, an open-source software package designed to aid in the rapid prototyping of 3D DNA-origami shapes. DNA nanotechnology allows for the self-assembly of complex structures using base-pairing specificity. The authors previously adapted the 'scaffolded DNA origami' method to create 3D shapes formed by pleated layers of double helices constrained to a honeycomb lattice. However, the design process was cumbersome and time-consuming. CaDNAno simplifies this process by providing a graphical user interface that facilitates the design of DNA sequences for folding 3D honeycomb-pleated shapes. The software is demonstrated through the design, assembly, and analysis of a series of rectangular-block motifs, which were found to be well-behaved building blocks for future studies. The use of caDNAno significantly reduces the effort required for designing 3D DNA-origami structures, and the software is available at http://cadnano.org/ with example designs and video tutorials. The source code is released under the MIT license.