Background: Cesarean scar pregnancy (CSP) can cause life-threatening complications for pregnant women, so early diagnosis and treatment are crucial for the prevention of maternal mortality. Transvaginal color Doppler ultrasonography, along with serum beta-human chorionic gonadotropin (β-hCG) level follow-up, can be a noninvasive and proper method for the post-treatment follow-up of a CSP. The present study investigated the Doppler resistance index (RI) changes in CSPs after medical management with methotrexate (MTX), evaluated by sac diameter and serum β-hCG level measurement. Methods: Overall, 28 patients with CSP and a history of previous cesarean delivery, a gestational age of less than 9 completed weeks at ultrasonography, the decision to medical management by the gynecologist, and having a β-hCG level before treatment were enrolled and initially examined by ultrasound and Doppler studies. The RI of the closest myometrial artery to the CSP mass and its diameter underwent measurement. After 72 hours, 1 week, and 2 weeks of medical therapy with MTX, the mentioned variables were measured again and compared with the values of the pretreatment time point. Results: RI values were significantly higher at all the follow-up time points compared with the pre-treatment time points, while serum β-hCG levels were significantly lower. Moreover, sac diameter values increased 72 hours post-treatment and then represented a decline. Conclusion: Serial Doppler ultrasound examination can be a useful method to evaluate the success or failure of the medical management of CSPs, as the increase in RI values occurs rapidly after medical treatment and correlates well with serum β-hCG levels.