Willem Janszoon Blaeu (1571 – 21 October 1638), also abbreviated to Willem Jansz. Blaeu, was a Dutch cartographer, atlas maker and publisher.
Blaeu was born at Uitgeest or Alkmaar. As the son of a well-to-do herring salesman, he was destined to succeed his father in the trade, but his interests lay more in Mathematics and Astronomy. Between 1594 and 1596 as a student of the Danish astronomer Tycho Brahe he qualified as an instrument and globe maker.
Once he returned to the Netherlands, he made country maps and world globes, and as he possessed his own printing works, he was able to regularly produce country maps in an atlas format, some of which appeared in the Atlas Novus published in 1635. In 1633 he was appointed map-maker of the Dutch East India Company. He was also an editor and published works of Willebrord Snell, Descartes, Adriaan Metius, Roemer Visscher, Gerhard Johann Vossius, Barlaeus, Hugo Grotius, Vondel and the historian and poet Pieter Corneliszoon Hooft. He died in Amsterdam.