Purpose: Increasing global migration, high obesity in developed countries, and ethnic health inequalities are compelling reasons to monitor immigrant obesity trends. Longitudinal studies of ethnicity, length of residence, and adiposity in contexts outside of the United States are lacking. Methods: Nine waves (2006–2014) of the Household Income and Labour Dynamics in Australia survey were analyzed (n = 20,934; 52% women; 101,717 person-year observations) using random effects modeling to assess average annual change in body mass index (BMI) by ethnic group. A second analysis used an immigrant only cohort (n = 4583; 52% women; 22,301 person-year observations) to examine BMI change by length of residence. Results: Over 9 years, mean BMI increased significantly in all ethnic and Australian-born groups, and by the final wave, mean BMI exceeded 25 kg m −2 for all groups. Trajectories of change did not vary between groups, with the exception of slower BMI increases for North-West European men compared with Australian born. Immigrants residing in Australia for 10–19 years had significantly faster annual increases in BMI compared with long-term immigrants (≥30 years). Conclusions: Immigrants to Australia, regardless of ethnicity, are at risk of obesity over time. Obesity prevention policy should prioritize immigrants in the early-mid settlement period.