"They've been separated for several months," a source close to the couple told People."This was a hard decision for both of them to make.
"The relationship just ran its course."
Diane, 48, married the True Grit star in 2004 in a ceremony held at his ranch in central California.
The couple were first introduced at a party in 2002 by Barbra Streisand, Josh's stepmother through her marriage to James Brolin. They have no children together.
The divorce will be the second for both. Josh, 45, was married to actress Alice Adair from 1988 until 1994, and they have two children, Trevor Mansur and Eden, while Diane was previously wed to Highlander star Christopher Lambert, the father of her daughter Eleanor Jasmine.