Willem Janszoon Blaeu (1571 – 21 October 1638), also known as Willem Jansz. Blaeu (Dutch pronunciation: [ˈʋɪləm ˈjɑnszoːn ˈblʌu];, was a Dutch cartographer, globe maker, and publisher. He was one of the leading figures of the Dutch school of cartography in the seventeenth century.

Trained in astronomy under Tycho Brahe between 1594 and 1596, Blaeu established a workshop in Amsterdam where he produced globes, nautical charts, and maps. These works formed the basis of a series of atlases, including the Atlas Novus (1635), and were widely used in navigation and geographic study.

In 1633 he was appointed mapmaker to the Dutch East India Company. After his death in 1638, his business was continued by his sons, including Joan Blaeu, who later published the multi-volume Atlas Maior, one of the most extensive atlases of the seventeenth century.

Willem Janszoon Blaeu was born in 1571 at Alkmaar or Uitgeest, in the Dutch Republic, the son of Stijntge and Jan Willemsz., a prosperous herring merchant. 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. He would also become a well established cartographer. 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 1600, Blaeu observed a star in the constellation Cygnus that had not previously been recorded. On 8 August of that year he measured its position using nearby stars, including Vega and Albireo, as reference points. The star initially appeared as a third-magnitude star but later decreased in brightness. It was later identified as a variable star and is now known as P Cygni.

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.

Willem Janszoon Blaeu died in Amsterdam on 21 October 1638. After his death, the publishing and cartographic business passed to his sons Joan Blaeu and Cornelis Blaeu, who continued their father's work. Cornelis died in 1644, after which Joan Blaeu carried on the enterprise. He later completed his father's atlas project, publishing the multi-volume Atlas Major, one of the most extensive atlases produced in the seventeenth century.