Background COVID-19 has become a major global health problem, and healthcare professionals are facing lot of pressure and stress. Accumulated resources and energy obtained via interpersonal relationships is called social capital, which can reduce the negative effects of pressure and stress related to the workplace by impacting happiness and moral courage. This study explored the effect of workplace social capital on moral courage and happiness in nurses working in the COVID-19 wards. Methods In this cross-sectional study, using a random sampling method, 169 nurses from three hospitals in East Mazandaran province, Iran, participated who worked in COVID-19 wards. The Onyx and Bullen Social Capital Questionnaire, the Sekerka's Moral Courage Scale, and the Oxford Happiness Inventory were used in this study. Descriptive analysis, Pearson correlation analyses, and stepwise multiple regression were performed for data analysis. Results The mean age of nurses was 31.38 ± 6.82 years. Socio-demographic factors such as age, gender, educational level, and employment status were significant predictors of workplace social capital. Social capital was positively correlated with moral courage (r = 0.29, p < 0.01) and happiness (r = 0.32, p < 0.01). In addition, social capital explained 6.8 and 8.6% variance in predicting moral courage and happiness, respectively. Conclusions Workplace social capital is a vital organizational phenomenon affecting nurses' moral courage and happiness, especially during the COVID-19 pandemic. Thus, hospitals should be aware of the importance of social capital; they should ensure that all the practices and policies are in place to develop and increase it.