Invasive species are considered an increasing global threat to the marine ecosystems. Understanding which factors can accelerate or reduce invasion success is one of the main goals of invasion ecology. The present study investigated the interactive effects of propagule pressure and native functional diversity on the invasion success of the invasive alga Sargassum muticum (Yendo) Fensholt over a 1-year invasion process (i.e. settlement, recruitment and colonisation). Invasibility of macroalgal assemblages with different functional diversity was assessed under high and low propagule pressure. Synthetic assemblages resembling natural marine macroalgal assemblages were built and used as model systems. Recruitment discs were used to record early settlement at the assemblages. Recruitment and colonisation success were monitored in the field 2 and 10-months after exposure to propagule rain at the laboratory. In addition, availability of resources (space and photosynthetic active radiation, PAR) was also monitored for each assemblage and used as predictive variables in the analyses. Results showed that the effects of propagule pressure and functional diversity varied over the different invasion stages. Contrasting to previous studies on S. muticum, space availability did not play a significant role explaining recruitment success of the invader. PAR, however, was a key resource in the invasion success of S. muticum. Our findings showed that the invasion success of S. muticum over the different invasion stages was influenced differently by recipient assemblage traits. Hence, invasion success may better be explained by the interaction of environmental factors that covary with species diversity and identity at different invasion stages.