Existing　data　and　models　suggest　increasing　prominence　of　dinoflagellates　and　their　blooms　in　future　warmer　ocean　but　supporting　long-term　data　are　sparse.　Here,　we　used　18S　rRNA　gene　sequencing　to　investigate　sedimentary　ancient　dinoflagellate　communities　in　northern　South　China　Sea　and　compared　them　with　contemporary　dinoflagellate　data　from　global　oceans　(TARA　Oceans　data)　and　40　years　of　dinoflagellate　bloom　records　in　China.　We　found　a　continuous　warming　(by　~4.3°C　in　mean　annual　sea　surface　temperature)　from　12　to　4.3　kiloyears　before　present　(kyr　BP),　which　caused　an　initial　increase　in　the　relative　abundance　and　diversity　of　dinoflagellates,　followed　by　a　decrease　reaching　the　lowest　value,　probably　due　to　thermal　stress.　However,　dinoflagellates　flourished　again　after　4.3　kyr　BP,　coinciding　with　a　rapid　increase　in　human　activities.　Further　analyses　indicated　that　warming　and　environmental　changes　during　the　Holocene　favored　dinoflagellate　generalists　over　specialists.　These　generalists　have　also　been　abundant　throughout　contemporary　low-　and　mid-latitude　regions,　whereas　specialists　were　more　abundant　at　higher　latitudes.　The　predominant　generalist　genera　Noctiluca,　Gymnodinium,　and　Prorocentrum　in　core　sediment　corresponded　to　taxa　responsible　for　most　dinoflagellate　blooms　in　the　contemporary　China　Seas　over　the　past　40　years.　The　success　of　generalists　during　warmer　periods　suggests　that　dinoflagellate　blooms　are　likely　to　expand　geographically　rather　than　simply　shift　toward　high　latitudes　under　global　warming.　Moreover,　the　homogenization　of　dinoflagellate　communities　resulting　from　generalist　expansion　may　significantly　reduce　the　complexity　of　marine　plankton　interactions　and　compromise　ecosystem　services　under　global　warming.　　©　2025　The　Author(s).