This randomized controlled trial will include 300 participants with diabetes mellitus type 2. Participants will be recruited from various settings, including diabetes outpatient treatment centers and internet forums/groups. Methods such as newspaper articles, flyers, posters, and media articles will be used to inform potential participants about the study (all material will be in German). Participants will be screened for inclusion and exclusion criteria via telephone, and randomly assigned to either (1) a treatment group that may use care as usual (CAU) and in addition immediately receives two-month access to the novel, internet -administrated intervention (Covivio), (2) an active control group that may also use CAU and in addition immediate receives two-month access to a psychological online-relaxation-training (Relaxio), or (3) a wait list control group, in which they receive CAU. Measurements are collected online at pre-treatment (T0), post-treatment (T1), and three (T2) and six months (T3) follow-up. After the last follow-up (T3), all participants gain access to the online-program which they had not yet been able to use.