This paper analyzes the effects of informal finance on formal finance in West African Economic and Monetary Union (WAEMU) countries. To this end, it uses a Probit model with a binary endogenous regressor and data from the Global Findex database, 2021. The findings indicate that having already saved informally positively affects the behavior of individuals to save formally. They also indicate that informal financial credit positively affects formal financial credits in the WAEMU. These findings indicate that formal finance is complementary to informal finance in the WAEMU. On the other hand, the findings show that labour force participation, mobile banking use and online banking use increase the likelihood of use of formal and informal services in the WAEMU. This paper suggests that formal financial institutions (mainly banks and microfinance institutions) should strengthen their collaboration with actors (money keepers, itinerant bankers, tontines systems) involved in the provision of informal financial services to populations in order to improve formal financial intermediation in the WAEMU.