The singer, 36, and actress, 37, had strongly defended their marriage in the wake of Thicke's controversial performance with Miley Cyrus at the VMAs last August and persistent rumors of infidelity.
But sources close to the couple, who started dating as teenagers and were married in 2005, say the marriage had been falling apart for a while.
"Their marriage was unravelling for a long time," a source said. Added another: "It has been a long time coming."
The pair has a 3-year-old son, Julian, and say they remain close.