Body composition is a crucial factor associated with the glycated hemoglobin (A1c), but most studies use proxy measures, such as body mass index. This study examined the association of body composition with A1c in Taiwanese adults with type 2 diabetes. A cross-sectional study was performed on participants aged ≥ 18 years completed the body composition assessment in the clinical diabetes center. Body composition and A1c were evaluated at baseline, 3 months and 6 months by multi-frequency bioelectric impedance analysis. Multiple linear regression analyses were used to examine their association with A1c. A total of 335 subjects were recruited, with an average age of 64.7 ± 11.3 years. After controlling confounders, changes in adipose tissue had the greatest influence on A1c. At the third and sixth months, for each unit increase in visceral fat area and body fat mass, A1c respectively increased most significantly by 0.34 units (P < .001) and 0.32 units (P < .001). During the study period, muscle mass indicators are inversely associated with A1c. visceral fat area and PBF are directly related to increases in A1c at 3 and 6 months, respectively, indicating that higher adiposity is associated with worse glycemic control.