Mediterranean marine biodiversity is undergoing a tropicalization process.We present the first record and spread dynamics of the tropical green seaweed Halimeda incrassata (Bryopsidales, Chlorophyta) fromthe Mediterranean Sea. Plants were observed at 2 sites offMallorca island (NW Mediterranean), and species identification was confirmed molecularly. Since the first observation of this alien seaweedin 2011, it has spreadrapidly inthe shallow subtidal habitat ofamarine protected area located in Palma Bay, which has a depth of up to ca. 20 m. By 2015, this species spread by a factor of 6.75 and extended to an area of 2.7 km2, covering a total of 41% of the monitored area. The dynamics of the invasion suggest rapid colonization and establishment and spreading of the species through shallow sedimentary and rocky habitats. H. incrassata has the potential to strongly alter the ecosystem services and functioning of shallow Mediterranean habitats. We encourage authorities to pay attention to the spread of this alien seaweed at larger scales and determine the positive and negative effects of the invasion in order to facilitate research- driven decision making.