Aims/Hypothesis We undertook phenotypic characterization of early-onset and late-onset type 2 diabetes (T2D) in adult Black African and White European populations with recently diagnosed T2D to explore ethnic differences in the manifestation of early-onset T2D. Methods Using the Uganda Diabetes Phenotype study cohort of 500 adult Ugandans and the UK StartRight study cohort of 714 White Europeans with recently diagnosed islet autoantibody negative type 2 diabetes, we compared the phenotypic characteristics of participants with early-onset T2D (diagnosed at <40 years) and late-onset T2D (diagnosed at ≥40 years). Results One hundred and thirty four adult Ugandans and 113 White Europeans had early-onset T2D. Compared with late-onset T2D, early-onset T2D in White Europeans was significantly associated with a female predominance (52.2% vs. 39.1%, p=0.01), increased body mass index (mean [95% CI]- 36.7 [35.2-38.1] kg/m2 vs. 33.0 [32.4-33.6] kg/m2, p<0.001), waist circumference (112.4 [109.1-115.6] cm vs. 108.8 [107.6-110.1] cm, p=0.06), and a higher frequency of obesity (82.3% vs. 63.4%, p<0.001). No difference was seen with the post-meal C-peptide levels as a marker of beta-cell function (mean [95% CI]- 2130.94 [1905.12-2356.76] pmol/L vs. 2039.72 [1956.52-2122.92], p=0.62). Conversely, early-onset T2D in Ugandans was associated with less adiposity (mean [95% CI] waist circumference- 93.1 [89.9-96.3] cm vs. 97.4 [95.9-98.8] cm, p=0.006) and a greater degree of beta-cell dysfunction (120-minute post-glucose load C-peptide mean (95% CI) level- 896.08 [780.91-1011.24] pmol/L vs 1310.10 [1179.24-1440.95] pmol/L, p<0.001), without female predominance (53.0% vs. 57.9%, p=0.32) and differences in the body mass index (mean [95% CI]- 27.3 [26.2-28.4] kg/m2 vs. 27.9 [27.3-28.5] kg/m2, p=0.29). Conclusions/Interpretation These differences in the manifestation of early-onset T2D underscore the need for ethnic-specific therapeutic and preventive approaches for the condition.