Women with a history of gestational diabetes mellitus are at risk of developing prediabetes. Using a cross-sectional analytic design, the factors related to prediabetes among Thai women (n = 210) with a history of gestational diabetes mellitus were investigated. The main outcomes measured were two-hour plasma glucose after taking a 75 g oral glucose tolerance test at a six week postpartum visit. Data were analyzed using descriptive statistics and Pearson's correlation. The study showed that the incidence of prediabetes among postpartum Thai women was 26.67%. Factors associated with prediabetes were: (i) being over 35 years of age; (ii) three or more pregnancies; (iii) recurrent gestational diabetes mellitus; (iv) high plasma glucose before taking a 100 g glucose tolerance test and high postprandial plasma glucose during pregnancy; and (v) being overweight or obese at six weeks' postpartum. The results showed that nursing interventions need to be implemented to reduce plasma glucose and body mass index in order to prevent or reverse prediabetes.