Sustainable fashion has developed as a response to the growing prominence and awareness of the negative environmental and social impacts of fashion apparel throughout its life cycle. Responses to these wide-scale impacts have focused on piecemeal strategies that lack a cohesive perspective. The notion of design thinking and a holistic viewpoint are increasingly being seen as valued strategies for developing a sustainable fashion system. Fashion designers generally lack the tools to enable change and are caught within a system that cannot fulfill the potential of design-driven solutions for sustainability. Transformations to the design process, business practices, consumer behaviours and supply-chain sustainability are needed. This dissertation presents a series of manuscripts investigating a re-conceptualization of fashion design for system sustainability. Concepts put forth in the first manuscript, Theorizing the Fashion System provide context for a design focus. This study reviews existing theories of fashion production and consumption, for the purpose of establishing a theoretical framework to support subsequent research and tool design. The second manuscript Tools for Sustainable Fashion Design: An Analysis of their Fitness for Purpose examines existing design tools developed specifically for sustainable fashion designers. This research led to the creation and proposal of two conceptual frameworks: an innovation framework and five-dimensional model of sustainable fashion. Using the frameworks to analyze the tools and sustainable strategies within the tools resulted in the identification of three tool archetypes: 1) Universal, 2) Participatory and 3) Assessment. The third manuscript investigates and analyzes current design practices of sustainable fashion micro and small enterprises (MSE) and available sustainable design tools. The fourth manuscript, The reDesign Canvas: Fashion Design as a Tool for Sustainability, is a qualitative in-depth case study with a small fashion start-up. Utilizing observations in the field, interviews and design sessions, this study was able to identify leverage points within the design process to integrate sustainable strategies. The data collected informed the development of a sustainable fashion design tool, the reDesign Canvas. This framework was tested and refined with the case study. This work aims to contribute a reconceptualization of the fashion design process to provide designers with the tools necessary to achieve a sustainable fashion system.