William Howard Jaco (born July 14, 1940 in Grafton, West Virginia) is an American mathematician who is known for his role in the Jaco–Shalen–Johannson decomposition theorem and efficient triangulations of 3-manifolds.

Jaco has held the distinguished positions of Regents Professor (2008)[3] and Grace B. Kerr Professor at Oklahoma State University where he was recognized as the 2017 Eminent Faculty Member and in 2019 recognized as Regents Distinguished Research Faculty member.

He has also held the Lois and Fred Gehring Distinguished Visitor Chair at University of Michigan.

To commemorate Jaco's 70th birthday and his career as a mathematician, Oklahoma State University held a conference in topology called Jacofest in June 2010.

His mathematical research is in Geometry and Topology where he studies low-dimensional manifolds, decision problems, algorithms and complexity theory.