Willem Janszoon Blaeu (1571 – 21 October 1638), also abbreviated to Willem Jansz. Blaeu, was a Dutch cartographer, atlas maker, and publisher. Along with his son Johannes Blaeu, Willem is considered one of the notable figures of the Netherlandish or Dutch school of cartography during its golden age in the 16th and 17th centuries.
Blaeu was a pupil of the great Danish astronomer Tycho Brahe, from whom he learned a very scientific approach to the field of cartography, which served to enhance his reputation. His early work concentrated on globe making, but by the early 17th century he began producing separately issued maps.
Blaeu founded his publishing firm in 1596, and with the collaboration of his sons, Cornelius (1616-1648) and Joan (1596-1673), it was the most productive cartographic establishment in the Netherlands until it was destroyed by the Great Fire of 1672.
Appointed mapmaker to the Dutch East India Company in 1633-1634, Blaeu had access to fresh geographical information that was not available to any of his contemporaries. He published his first world atlas, the Atlantis Appendix, with 60 maps in 1630, and continued to produce new maps at such a rate that by 1634, he abandoned the single volume format and announced his intention to publish a new world atlas, entitled the Theatrum, which eventually grew into the monumental 11-volume Atlas Maior, completed by his son Joan in 1662.