William Ivan Martin Jr. (March 20, 1916 – August 11, 2004) was an American educator, publishing executive, and author of more than 300 children's books. His notable works include The Sounds of Mystery, Chicka Chicka Boom Boom (co-authored with John Archambault), Brown Bear, Brown Bear, What Do You See?, Baby Bear, Baby Bear, What Do You See?, Panda Bear, Panda Bear, What Do You See?, and Polar Bear, Polar Bear, What Do You Hear? (all collaborated with illustrator Eric Carle).

Martin was not only a prolific writer but also an elementary-school principal, teacher, and poet. He held a doctoral degree in early childhood education and his ability to speak directly to children is reflected in his bestselling classics.

Born in Kansas, Martin worked as an elementary-school principal in Chicago before moving to New York City, where he worked in publishing and developed innovative reading programs for schools. Eventually, he devoted himself full-time to writing children's books. He lived in New York until 1993, after which he moved.

In his honor, the Bill Martin Jr. Award, the Kansas state award for best children's picture book, was established in 1996.