Objective: This study examines how changes in cohabitation or marital status affect Body Mass Index (BMI) over time in a large representative sample. Method: Participants were 20,950 individuals (50% female; 19 to 100 years), representative of the German population, who provided 81,926 observations over 16 years. Face-to-face interviews were used to obtain demographic data, including cohabitation and marital status, height, body weight, and weight-relevant behaviors (exercise, healthy eating, and smoking). Control variables included age, notable changes in status (life events such as having children or change in employment status), perceived stress, and subjective health. Results: Cohabitation led to significant weight gain in men and women—after four years or longer, about twice the gain associated with marriage (controlling for weight-related behaviors, age, children, employment, stress, and health). BMI after separation was largely comparable to BMI before starting cohabitation; women lost some weight in the first year, men gained some weight after four or more years of separation. Divorce generally predicted weight gain. Changes in exercise, healthy eating, and smoking did not attenuate the effect of changes in relationship status on BMI. Conclusions: This is among the first longitudinal studies to directly compare the effects of key changes in relationship status on BMI. The findings extend and qualify previous results by showing that the benefits of marriage or cohabitation do not necessarily include a healthier BMI. They also suggest that relationship transitions—particularly moving in with a partner and divorce—may be important time windows for weight gain prevention.