We aimed to evaluate the diagnostic accuracy of serum D-dimer levels in pregnant women with adnexal torsion (AT). The pregnant women with ovarian cysts who suffered from pelvic pain were divided into two groups; the first group consisted of the cases with surgically proven as AT (n = 17) and the second group consisted of the cases whose pain were resolved in the course of follow-up period without required surgery (n = 34). The clinical characteristics and serum D-dimer levels were compared between the groups. Patients with AT had a higher rate of elevated serum white blood cell (WBC) count (57% vs. 16%, p = 0.04) and serum D-dimer levels (77% vs. 21%, p < 0.01) on admission in the study group than in the control group. Elevated D-dimer and cyst diameter larger than 5 cm yielded highest sensitivity (82% for each); whereas the presence of nausea and vomiting and elevated CRP had the highest specificity (85% and 88%, respectively). This is the first study that evaluates the serum D-dimer levels in humans in the diagnosis of AT, and our findings supported the use of D-dimer for the early diagnosis of AT in pregnant women.