In recent years, a growing number of organizations have provided flexible work arrangements (FWAs) for employees to facilitate their work-life balance. However, to what extent the use of FWAs shapes gender inequality in housework remains debated. Using longitudinal couple-level dyadic data in the United Kingdom (2012–2020), this study shows that among heterosexual couples, women’s housework hours are approximately 3.5 times longer than men’s housework hours. Women’s use of FWAs significantly intensifies gender inequality in housework hours regardless of whether their husbands/partners use FWAs or not. In contrast, men’s use of FWAs does not change the unequal gendered division of housework even when their wives/partners do not use any FWAs. The pattern is particularly pronounced in routine housework tasks such as cooking, washing, and cleaning and mainly applies to the reduced hours and teleworking arrangements. Future research should pay more attention to the unintended consequences of flexible working on gender inequality within families.