Grandparental support is positively associated with women's labor force participation and fertility transitions. Specifically, grandparental support shapes adult child's entry into parenthood by influencing adult child's expectations about future care. In this framework, the current study investigates whether observed grandparental support impacts adult sons' and adult daughters' fertility intentions in a lowest-low fertility country such as Spain. We use data from the new wave of the Spanish Fertility Survey (2018), and we find that receiving grandparental support has a positive and significant impact on women's but not on men's fertility intentions. Furthermore, this impact remains positive after controlling for women's socio-economic characteristics. Finally, education results the main mediator for the relationship between grandparental support and women's fertility intentions, with highly educated women displaying a significantly increase of their fertility intentions compared to their low-educated counterparts. Our findings highlight gender differences in the perceived role of grandparental support for future fertility, and the importance of informal care for balancing family and work in the Spanish context.