Background: Parenthood can reduce marital satisfaction (MS). Problem-solving skill (PSS) training enables couples to solve family problems and improve MS. Objectives: We aimed to evaluate the effectiveness of PSS training on MS of primigravida breastfeeding mothers. Methods: The research design of this randomized controlled field trial study was pretest-posttest with a control group done on 61 primigravida breastfeeding mothers who attended healthcare centers affiliated to Babol University of Medical Sciences, Iran. They were allocated to two experimental (30 individual) and control (31 individual) groups using the blocked randomized allocation technique based on pretest scores of MS. The small group program (10 - 11 women) of PSS training was performed for six 80-minute sessions for the experimental group. The control group did not receive the intervention. All participants completed the ENRICH inventory three times, namely before, immediately after, and one month after the intervention by self-report. The level of significance was 0.05. Results: Total MS score in the experimental group increased from 61.70 ± 12.49 before intervention to 70.93 ± 11.10 immediately after, and 72.64 ± 10.93 one month after the intervention significantly. Moreover, except for the score of the idealistic distortion dimension, the mean scores of the other dimensions of the ENRICH increased significantly over time in the experimental group. Conclusions: PSS training can be used as an effective method to improving MS in primigravida breastfeeding mothers. Therefore, this intervention is recommended as an effective program for improving MS.