Widespread plant species exhibit considerable intraspecific variation due to phenotypic plasticity and genetic adaptation. Portulaca oleracea, a ubiquitous invasive annual C4 weed, poses a challenge to agriculture worldwide. Its resilience to climate change, demonstrated by its ability to tolerate extreme temperatures and high salinity, suggests an increasing invasiveness that threatens agricultural yields. Despite its short life cycle, P. oleracea produces numerous seeds that can thrive in extreme environments. In this study, eight Tunisian wild populations of P. oleracea were analyzed to understand seed morphology and germination under different salinity conditions. Under control conditions, a large variability in the studied parameters and a geographical differentiation primarily related to temperature and latitude were found, indicating a clinal adaptation across an aridity gradient. In addition, our results showed that populations from Saharan and arid regions exhibited greater salt resistance compared to populations from semi-arid regions, demonstrated by larger seeds and robust germination. These results emphasize the joint role of clinal adaptation and phenotypic plasticity in the invasiveness of P. oleracea in different environments. Utilizing these findings could lead to more effective strategies for managing the spread of the plant in agricultural landscapes.