Obesity modifies the association between sodium and potassium intake and blood pressure. However, the impact of obesity on the relationship between the sodium-potassium balance and cardiovascular risk in type 2 diabetes (T2DM) patients remains unclear. We investigated the relationship between the 24-h urinary sodium-to-potassium ratio and serum asymmetric dimethylarginine (ADMA) level, which is a cardiovascular risk factor, in Japanese T2D patients with or without obesity. This cross-sectional study included 243 patients with T2DM who were hospitalized for diabetes education. Urinary sodium-to-potassium ratios were calculated from 24-h urine collection. The subjects were divided into two groups according to their BMIs (<25 and ≥25). The serum ADMA levels positively correlated with the urinary sodium-to-potassium ratios in non-obese patients, but not in obese patients. Multivariate linear regression analyses showed that the serum ADMA levels positively correlated with the urinary sodium-to-potassium ratios after adjustment for other confounders in non-obese patients. This correlation was observed in non-obese patients with hypertension, but not in those without hypertension. Measurement of the pulse wave velocity and intima-media thickness revealed that elevation of the serum ADMA levels was significantly associated with an increase in the degree of atherosclerotic progression in subjects with T2DM. Additionally, an assessment of dietary intake showed that the consumption of dairy products as well as of green and yellow vegetables was negatively associated with urinary sodium-to-potassium ratios in patients with T2DM. In conclusion, elevation of serum ADMA may be positively associated with the urinary sodium-to-potassium ratio in Japanese non-obese patients with T2DM.