Time series with in-situ data of the Poseidon system for the monitoring of the Greek Seas. Data include atmospheric (such as air temperature, wind speed and direction, waves, currents) and marine physical and biochemical parameters (such as temperature, salinity, chlorophyll-a, dissolved oxygen, turbidity). Data have been collected by several measuring platforms like fixed buoys, Argo floats, oceanographic cruises, FerryBox systems. They have been subject to delayed mode quality control and expand from 25/5/2000-27/5/2020.