The craft of improvisational quilting involves working without the use of a predefined pattern. Design decisions are made “in the fabric,” with design experimentation tightly interleaved with the creation of the final artifact. To investigate how this type of design process can be supported, and to address challenges faced by practitioners, this paper presents PatchProv, a system for supporting improvisational quilt design. Based on a review of popular books on improvisational quilting, a set of design principles and key challenges to improvisational quilt design were identified, and PatchProv was developed to support the unique aspects of this process. An evaluation with a small group of quilters showed enthusiasm for the approach and revealed further possibilities for how computational tools can support improvisational quilting and improvisational design practices more broadly.