Johannes Dou, also known as Jan Jansz., or Douw (Leiden, February 7, 1615 - March 5, 1682) was a land surveyor who mainly worked for the Rijnland Water Board.

Johannes Dou was born in Leiden in 1615 (according to his father on 7 February) as the son of Jan Pieterszoon Dou and Josina Sadelaer. On March 8, 1634, Johannes was enrolled at Leiden University as a student of mathematics.

He attended lectures with Frans van Schoten sr., professor of 'Duytsche mathematicque', the training for engineers, fortress builders and land surveyors. His father was a prominent land surveyor with a large production, who also published several books about his field.

He probably taught his son what he needed in practice. In any case, on September 10, 1635, Johannes was admitted by the Court of Holland to practice the profession of land surveyor. In 1637 he was appointed notary.

After the death of his father in 1635, Dou was appointed together with Steven van Broeckhuizen as land surveyor of Rijnland and from 1637 also appears as a notary in Leiden. Like his father, he was extremely productive, but never attained his scientific stature.

In addition to many small commissions, he also distinguished himself with some more tasteful works. For example, in 1661 he designed the canal from Leiden to Utrecht, he was deployed in 1672 during the campaign against the French and in 1675 he completed his large map of the Uitwaterende Sluizen.