Posidonia oceanica (L.) Delile is a seagrass endemic to the Mediterranean Sea and a significant contributor to human well-being through a variety of ecosystem functions, such as primary productivity and carbon cycling. Its widespread presence over the coasts of many Mediterranean countries, its role as a biodiversity hotspot and the various ecosystem services it provides have led to the recognition of P. oceanica meadows as a Marine Priority Habitat under the EU Habitats Directive. Despite its ecological significance, most methods to assess ecological functions in this seagrass species are either destructive or expensive, especially when estimating leaf biomass and primary productivity. Our work aims at contributing to the development of an effective method to estimate leaf biomass based on morphometric parameters. In this study, 2500 individual leaves from 351 shoots of Posidonia oceanica were collected, across 16 sites in the Ligurian Sea, over two time periods (2016–2018 and 2024–2025), and analyzed for total leaf area and dry weight. An allometric equation following a power-law structure was derived using linear mixed-effects models and subsequently validated via 10-fold cross-validation. This equation allows the use of area as a predictor of leaf weight, with an absolute error of ±20.8%. Although some variations in the intercept term were observed, the allometric scaling structure remained consistent across space and time, providing the first robust species-specific allometric tool to estimate P. oceanica biomass. This work forms the basis of a non-destructive protocol for meadow-scale biomass estimation. Its application has the potential to bring significant improvements in the collection of data regarding P. oceanica carbon fixation capacity and other aspects of its natural capital value.