While the practice of polygyny is common in Ghana, little is known about its impact on the use of reproductive health services. The aim of this study was to assess the relationship between polygynous marriage and the utilisation of skilled antenatal care (ANC), assisted skilled birth, and modern contraceptive services among married women in Ghana. Secondary data from the 2017 Ghana Maternal Health Survey were used for this study. The study included a weighted sample of 9,098 married women aged 15–49 years. We used multivariable logistic regression models to assess the association between polygyny and each outcome variables. Sensitivity analysis was conducted to assess the dose–response relationship between polygyny and each outcome variable. The prevalence of eight or more ANC contacts, assisted skilled births, and use of modern contraception were 47.0%, 81.4%, and 25.4%, respectively. The prevalence of women in polygynous marriages was 15.3%. Compared to monogynous marriage, polygynous marriage was associated with 19% lower odds of having eight plus ANC contacts (adjusted odds ratio [aOR] 0.81, 95% CI: 0.69, 0.96), 25% lower odds of having assisted skilled birth (aOR 0.75, 95% CI: 0.63, 0.89), and 19% lower odds of modern contraceptive utilisation (aOR 0.81, 95% CI: 0.66, 0.99). Interventions on reproductive health may need to prioritise women in polygynous marriages in order to improve the utilisation of skilled ANC, assisted skilled birth, and modern contraceptive services.