This study investigates competitive balance in five major European women’s soccer leagues—Women’s Super League (WSL) (England), Première Ligue (France), Frauen-Bundesliga (Germany), Toppserien (Norway), and Damallsvenskan (Sweden)—over the period 1997 to 2023. Using the Herfindahl-Hirschman Index (HHI), its deviation (dHHI), and a fixed-effects regression model with league and year controls, the analysis explores the effects of salary caps, league expansions, promotion/relegation systems, professionalization, and participation in the Union of European Football Association (UEFA) Women’s Champions League (UWCL). The results show that salary caps and league expansions enhance competitive balance, while promotion/relegation and an increase in player talent concentration reduce parity. The impact of professionalization and UWCL participation is insignificant. These findings provide valuable policy recommendations for promoting parity and supporting the sustainable development of women’s soccer leagues.