Gestational diabetes mellitus (GDM) is a pregnancy complication affecting many women, requiring changes in behaviours, which command them to learn self-care practices shortly. Digital interventions have been developed to support women with GDM. However, they have often overlooked women's needs and characteristics and failed to frame self-care theories into their design. To address this issue, we adopted a mixed methods approach to develop and refine a user-centred, evidence-based digital Toolkit for supporting self-care in GDM, providing behavioural and educational content, particularly about nutrition. To inform the development and refinement of the Toolkit, we conducted a literature review, observed sixty-six nutrition appointments, interviewed eleven dietitians and seventeen patients, and held co-creation sessions with two dietitians, all of which were analysed using a deductive Thematic Analysis. To validate the Toolkit, we conducted a survey with seventeen healthcare professionals, which was analysed using descriptive statistics. The final version of the NUTRIA Toolkit consists of four main modules with thirty-eight artefacts, including behavioural tools to assist women in GDM management. Despite some limitations, this study robustly endorsed the development and refinement of a user-centred, evidence-based Toolkit for supporting self-care in GDM, aiming for future feasibility and trial testing.