Disparate population surveys of type 2 diabetes mellitus (T2DM) have been conducted in Tonga for 4 decades. This study standardizes these surveys to enable assessment of T2DM and obesity trends in Tongans aged 25 to 64 years over 1973-2012, and projects T2DM prevalence to 2020 based on demographic and population weight changes. Eight surveys were standardized to the nearest census to produce nationally representative estimates. Linear period trends and prevalence projections to 2020 were produced using random-effects meta-regression. Over 1973-2012, T2DM prevalence increased from 5.2% to 19.0% (1.9%/5 years) and obesity prevalence from 56.0% to 70.2% (2.7%/5 years). T2DM prevalence period projection to 2020 is 22.3%. Based on modeling using body mass index, T2DM prevalence in 2020 could have been 12.7% and 16.8% in 2020 had mean population weight been 1 to 4 kg lower than 2012 levels.