It was established by Leiden University in 1633, to house the quadrant of Snellius, and is the oldest operating University observatory in the world (before this, astronomy taught at medieval universities tended to be of a more theoretical nature, and any observations were usually done with private equipment rather than at University observatories —see this timeline).
In Richard Proctor's 1867 map it is called then Kaiser Sea (after Frederik Kaiser of the Leiden Observatory).
Leiden | Leiden University | Royal Observatory, Greenwich | observatory | Royal Observatory | Paris Observatory | Palomar Observatory | Yerkes Observatory | Arecibo Observatory | Siding Spring Observatory | Mount Wilson Observatory | Smithsonian Astrophysical Observatory | Observatory | Harvard College Observatory | Ondřejov Observatory | Lowell Observatory | Kew Observatory | Roque de los Muchachos Observatory | Pulkovo Observatory | McDonald Observatory | Jodrell Bank Observatory | Chandra X-ray Observatory | Solar Dynamics Observatory | National Astronomical Observatory of Japan | Las Campanas Observatory | European Southern Observatory | Allegheny Observatory | Vienna Observatory | United States Naval Observatory | Parkes Observatory |