Trust and credibility are significant in digital health. Several studies have confirmed the influence of interface design on trust and credibility; however, further research is needed to understand how design explicitly impacts these two dimensions. Therefore, identifying what design principles contribute to trust and credibility is paramount to this field. Through a Delphi study with [N = 10] experts in User Interface and digital health, we identified and validated [N = 128] interface design principles. These principles are organised into a multidimensional framework comprising Visual, Information Architecture, Interaction, Social Presence, and Experience dimensions. Our findings indicate that while all dimensions contribute, Information Architecture and Experience dimensions yielded the highest number of “Extremely influential” principles, signifying their critical role in fostering trust and credibility in digital health.