Background: An unplanned pregnancy may follow by increasing depression and anxiety and the aim of the present study was to evaluate the mediating role of partner’s emotional reaction to pregnancy (PERP) on the relationship between pregnancy planning and prenatal mental health. Methods: This cross-sectional study was conducted on 303 healthy Iranian pregnant women during their third trimester. The levels of depression and anxiety were measured using the Edinburgh Postnatal Depression Scale and the State-Trait Anxiety Inventory. Further, the PERP score was measured using a researcher-made questionnaire. The data were analyzed using the plug-in application PROCESS macro. Results: The results showed that PERP score was related to pregnancy planning and prenatal depression and anxiety reversely. The direct effect of the pregnancy planning on depression (c=-.05) and anxiety levels (c= -.02) were not significant; but indirect effect of pregnancy planning on depression (Point Estimate= -.379, CI: -.523 to -.250) and anxiety levels (Point Estimate=-.560, CI: -.741 to -.385) with mediating role of PERP. were significant. Conclusions: The results indicated that the effect of the pregnancy planning on prenatal mental health is mediate by PERP and in unplanned pregnancy, women need to receive their positive partner’s reaction toward pregnancy to preserve their mental health.