Simon Antoine Jean L'Huilier (or L'Huillier) (Geneva, 24 April 1750 – Geneva, 28 March 1840) was a Swiss mathematician of French Hugenot descent. He is known for his work in mathematical analysis and topology, and in particular the generalization of Euler's formula for planar graphs.

He won the mathematics section prize of the Berlin Academy of Sciences for 1784 in response to a question on the foundations of the calculus. The work was published in his 1787 book Exposition elementaire des principes des calculs superieurs. (A Latin version was published in 1795.) Although L'Huilier won the prize, Joseph Lagrange, who had suggested the question and was the lead judge of the submissions, was disappointed in the work, considering it "the best of a bad lot." Lagrange would go on to publish his own work on foundations, and Augustin Louis Cauchy would later create the definitive approach.

He was elected in May, 1791 a Fellow of the Royal Society.

Note that this surname is sometimes rendered as Lhuilier or Lhuillier.