Ludolph van Ceulen (28 January 1540 – 31 December 1610) was a German / Dutch mathematician from Hildesheim. Like many Germans during the Catholic Inquisitions, he emigrated to the Netherlands.

Ceulen moved to Delft to teach fencing and mathematics. In 1594 he opened a fencing school in Leiden. In 1600 he was appointed the first professor of mathematics at Leiden University. He died in Leiden.

Ludolph van Ceulen spent a major part of his life calculating the numerical value of the mathematical constant π, using essentially the same methods as those employed by Archimedes some seventeen hundred years earlier. He published a 20 decimal digit value in his 1596 book Van den Circkel ("On the Circle"), later expanding this to 35 decimals. After his death, the "Ludolphine number",

3.14159265358979323846264338327950288...,

was engraved on his tombstone in Leiden. The tombstone was later lost but was restored in 2000.