Arabian Gulf ecosystems, particularly the phytoplankton communities, are influenced by natural and human activities. We analyzed coastal sediments chronology and pigment records to explore shifts in phytoplankton in the western Arabian Gulf, where long-term records are lacking. Our results revealed significant shifts in phytoplankton abundance over the past five decades. The stability of zeaxanthin concentration, indicated predominance of cyanobacteria from 1800 to the1960s, followed by a subsequent decline, potentially linked to industrialization and impactful events like the Gulf War oil spill. Since the early 1980s, the abundance of diatoms and dinoflagellates has increased, marking a transition from cyanobacteria and prasinophytes. This shift has resulted in an increase in the trophic status index since the 1970s and 1980s, indicating growing eutrophication coinciding with accelerated local development and urbanization. These findings highlight long-term changes in phytoplankton dynamics in the Arabian Gulf, emphasizing recent eutrophication processes.