Mac Barnett is an American writer of children's books living in Oakland, California. He was born in Castro Valley, California but grew up in Castro Valley and Oakland.

Barnett graduated from Pomona College, where he studied under the writer David Foster Wallace.

Extra Yarn, a picture book illustrated by Jon Klassen, won the 2012 Boston Globe–Horn Book Award and 2013 E. B. White Read Aloud Award. It was a Caldecott Medal Honor Book. Sam & Dave Dig a Hole, illustrated by Jon Klassen, won a Caldecott Honor and the 2015 E.B. White Read Aloud Award.

He is the co-creator, with Jon Klassen, of the Emmy-award winning children's animated series Shape Island on Apple TV+, based on their Shapes trilogy of picture books.

In 2011, Barnett wrote the Picture Book Proclamation, an art manifesto about picture books signed by 21 other children's writers.

In 2024, Barnett wrote La Porta Segreta, a book for adults about children's books, published in Italian by Terre di Mezzo.