The first to use the decimal point was a German Jesuit, Christopher Clavius. However, it was popularized by the Scottish mathematician, John Napier.
During the 16th century, Claudius was the most prolific German scholar in propagating mathematical knowledge. He published versions of other people's works, including Euclid's "Elements." He also invented several devices such as an instrument that measures angular fractions.
Napier is widely credited with bringing the decimal point into common practice, although he did not invent it. He was the first to use the decimal point as a separator between whole numbers and fractional numbers. Napier is most famous for inventing logarithms.