While standardized assessment of knowledge, attitudes, and practices (KAP) related to gestational diabetes and hypertension is possible with a valid tool, existing research remains limited. This prospective validation study aimed to develop and validate a novel tool to assess the KAP of midwives and obstetric nurses. We included 125 midwives and obstetric nurses who routinely care for patients with gestational diabetes and hypertension. The tool demonstrated good internal consistency (Cronbach’s alpha): knowledge (0.729, 95% CI, 0.654–0.776), attitude (0.756, 95% CI, 0.690–0.814), and practices (0.925, 95% CI, 0.905–0.943). Difficulty indices (d) ranged from 0.38 to 0.99 (knowledge), 0.41 to 0.99 (attitudes), and 0.41 to 0.93 (practices), indicating appropriate item difficulty. Discrimination indices (D) confirmed items could differentiate between respondents with low and high knowledge levels (D range: 0.02–0.77 for knowledge, 0.06–0.64 for attitudes, 0.20–0.84 for practices). The robust psychometric properties of this tool support its use in future research on KAP related to diabetes and gestational hypertension management in midwives and nurses. This instrument has the potential to be valuable in various settings, including baseline assessment before educational programs or evaluation of learning outcomes after interventions.