Seagrass meadows have been declining due to climate change and anthropogenic activities and, therefore, their restoration has been significantly increased worldwide. In Posidonia oceanica, endemic to the Mediterranean Sea, the use of seedlings in restoration actions have been lately encouraged, also due to the increase of sexual restoration of this plant associated to the increase of temperature. However, to successfully use seedlings in restoration, understanding the factors that affect their growth has become pivotal. This study aimed at evaluating the development of P. oceanica seedlings during the early life stages at i) different thermal environments by cross-transplantation between the eastern and western coast of Sardinia (Italy) and at ii) several P. oceanica canopy densities. Plant morphology (total number of leaves, total leaf length, maximum root length, number of roots, and tot necrosis length) and leaf fiber content were considered to evaluate seedling performance.Overall, results about survivorship and acclimation of seedlings after the cross-transplantation suggest that the different average thermal conditions of donor and receiving sites do not affect seedling development during the first months, considering that seedlings from the same origin developed consistently in both sites, highlighting a good acclimation. Furthermore, seedlings germinated in dead matte (0 % canopy density) developed longer roots than seedlings germinated in higher canopy density, likely due to the need of anchoring in an environment where there is no meadow to protect the seedlings from hydrodynamics. Seedling mortality was higher at high canopy density rather than low density or dead matte: even though P. oceanica meadows have an important nursery role, the conditions created within the meadow seem not to favor the seedling development, likely due to the low irradiance. Results of this study provide insights for future P. oceanica restoration actions using seeds and seedlings.