Background: The identification and management of depression during pregnancy is an important public health issue. Although many existing psychological intervention programs are effective, their implementation is plagued by issues, such as insufficient professional resources and lengthy intervention cycles. Studies have suggested that internet-based problem management plus (IPM+) can effectively address the aforementioned challenges in the management of general depression. However, its application in the pregnant population remains to be verified. Objective: The objective of our study was to explore the effect of IPM+ on depressive symptoms in pregnant women. Methods: From April to October 2024, 80 pregnant women were recruited from the Hubei General Hospital and randomly assigned to the intervention group (n=40) and the control group (n=40). The control group received routine care, while the intervention group received a 5-week IPM+ intervention (1.5 hours per week) in addition to routine care. The primary outcome measure is depressive mood, and the secondary outcome measures include anxiety symptoms, perceived stress, and sleep quality. The data were collected using the Edinburgh Postnatal Depression Scale, Pregnancy Anxiety Questionnaire, 4-item Perceived Stress Scale, and Insomnia Severity Index at baseline (T1), immediately after the intervention (T2), and at 3 months (T3). Results: Compared with the control group, the intervention group showed statistically greater improvements in depressive symptoms (between-group effect size: Hedges =0.74, 95% CI 0.23-1.24 at T2; Hedges =0.76, 95% CI 0.25-1.28 at T3), anxiety symptoms (between-group effect size: Hedges =0.51, 95% CI 0.03-0.98 at T2; Hedges =0.52, 95% CI 0.04-0.99 at T3), and perceived stress (between-group effect size: Hedges =0.71, 95% CI 0.23-1.18 at T2; Hedges =0.62, 95% CI 0.15-1.11 at T3) at both T2 and T3. In terms of sleep quality, the intervention group demonstrated statistically greater improvement compared with the control group only at T3 (between-group effect size: Hedges =0.68, 95% CI 0.19-1.17). Conclusions: IPM+ effectively alleviates depression and anxiety symptoms, reduces perceived stress, and improves sleep quality in pregnant women with antenatal depression. The intervention effects are sustained for up to 3 months postintervention. Trial Registration: Chictr.org.cn ChiCTR2400083324;