Reproduction, arguably the most important life-history trait with respect to evolutionary fitness, demands significant resources. The nutrients needed to reproduce are amongst the most critical of resources, but whether and how reproductive investment in animals is affected by nutrient limitation is largely unclear. We address this key knowledge gap by experimentally manipulating dietary availability of phosphorus (P) for reproductively mature Potamopyrgus antipodarum, a New Zealand freshwater snail. P is critical across life as a major component of nucleic acids and ribosomes but is often limiting in natural environments. We quantified multiple components of reproductive fitness relative to body-P content across life-history stages. Our study revealed a suite of outcomes linking key aspects of reproductive fitness to P availability - suggesting the central importance of P to life-history milestones. Together, our study demonstrates that P is a major component of multiple metrics of reproductive investment in a female animal system.