Abstract Background A healthy lifestyle can help prevent diseases that impair quality of life and lead to premature death. The Techniker health insurance fund offers a comprehensive online health program to support users in achieving their health goals of Increasing Fitness, Losing and Maintaining Weight, or Smoking Cessation. Methods The aim of this study is to test the long-term effectiveness of the web-based TK-HealthCoach with regard to the primary outcomes of increased physical activity, sustainable weight reduction, and smoking abstinence. We are conducting three interconnected, randomized controlled trials (RCT), one for each health goal, within which participants are allocated to an intervention group (interactive online health program) or a control group (non-interactive online health program). The effects of the intervention groups compared to the control groups will be analyzed by multi-level models for change. Participants’ data are captured via online questionnaires before the program starts (baseline t0), again when it ends (t1), and later at two follow-up surveys (t2 and t3); the latter 12 months after t1. We are documenting socio-demographic, health-related, and psychological variables as well as usage behavior data of the programs. According to our sample size calculation, we have to enroll 1114 participants in each Losing and Maintaining Weight and Increasing Fitness RCT and 339 participants in the Smoking Cessation RCT. Additionally, 15–20 participants in the interactive smoking-cessation program will be invited to qualitative telephone interviews with the aim to obtain detailed information concerning utilization, compliance, and satisfaction. The online RCTs’ inclusion criteria are: adults of each gender regardless of whether they are insured with Techniker health insurance fund. Persons with impairments or pre-existing conditions require a medical assessment as to whether the program is suitable for them. Specific exclusion criteria apply to each program/RCT. Discussion We assume that study participants will improve their health behavior by using the offered online health programs and that each health goal’s intervention group will reveal advantages regarding the outcome variables compared to the control groups. Study enrollment started on January 1, 2020. Trial registration German Clinical Trials Register, Universal Trial Number (UTN): U1111-1245-0273. Registered on 11 December 2019