Seminal fluid proteins (SFPs) are components of the ejaculate that often induce drastic changes in female physiology, such as reducing remating rate or shortening longevity. There is compelling evidence that these functions make SFPs a determinant of male reproductive success, and some evidence that males can strategically invest in their differential production. However, SFP-mediated effects have received relatively little attention in simultaneous hermaphrodites, that is, organisms that are male and female at the same time. Since this reproductive mode is widespread in animals and their SFPs have unique functions compared to separate-sexed species, examining SFPs of hermaphrodites would help generalize our understanding of the impact of SFPs. We therefore examined if individuals strategically alter seminal fluid production in response to mate availability and sperm competition in the freshwater snail Lymnaea stagnalis. We exposed snails to different social group sizes, and measured the expression of 6 SFP genes. We found that the snails plastically elevated SFP expression in the presence of at least 1 mating partner. Specifically, paired snails showed higher SFP expression than isolated snails, whereas SFP expression of snails exposed to sperm competition, that is, in a larger group size, was equivalent to that of paired snails. Furthermore, 5 out of 6 SFP genes we examined responded to mate availability in a very similar way, implying that overall seminal fluid production increases when the snails have mating opportunities. The plastic expression of seminal fluid depending on mate availability supports that SFPs play important roles in post-copulatory processes in this hermaphroditic species.