Evidence-based health information is a prerequisite for patients with type 2 diabetes to engage in self-management and to make informed medical decisions. The Internet is an important source of health information. In the present study, we systematically assessed formal quality, quality of decision support and usability of German and English language websites on type 2 diabetes. The search term ‘type 2 diabetes’ was entered in the two most popular search engines. Descriptive data on website quality are presented. Additionally, associations between website quality and affiliation (commercial vs. non-commercial), presence of the HON code quality seal and website traffic were explored. Forty-six websites were included. Most websites provided basic information necessary for decision-making, while only one website also provided decision support. Websites with a HON code had significantly better formal quality than websites without HON code. We found a highly significant correlation between usability and website traffic and a significant correlation between formal quality and website traffic. Most websites do not provide sufficient information to support patients in medical decision-making. Our finding that usability and website traffic are tightly associated is consistent with previous research indicating that design is the most important cue for users assessing website credibility.