In documents filed in L.A. County Superior Court, Simpson cites "irreconcilable differences" as the reason for the divorce.
She's asking for joint legal custody and primary physical custody of two-year-old Bronx.
Simpson, who is repped by disso queen Laura Wasser, is also asking for spousal and child support. Wentz is repped by Stacy Phillips.
Sources tell TMZ there is no prenup. The couple married in May, 2008.
The couple just released a joint statement, saying, "After careful consideration, we have made the difficult decision to file for divorce.
We remain friends and deeply committed and loving parents to our son Bronx, whose happiness and well-being remains our number one priority. We ask that everyone honor our privacy as we navigate this next phase of our lives."
We're told Pete agrees with Ashlee -- no third parties were involved. According to one connected source, "After Bronx was born Ashlee made a conscious decision to get out of the business, but Pete's heart was in music so he kept touring and that became a big issue in their relationship."
A conflict has already emerged, because sources tell TMZ Pete wants joint legal and physical custody of Bronx. In Ashlee's divorce papers, she's asking for primary physical custody.
TMZ just broke the story that Ashlee has filed for divorce.
Our sources say, no third party is involved. One source very familiar with the situation tells TMZ, "It's really all about Pete's erratic behavior." The source adds, "She really wanted out."
The divorce move is surprising -- just a few days ago they appeared to be one happy family during an outing in West Hollywood.
According to a rep for the Fall Out Boy band member, 1-year-old Bronx Mowgli Wentz was riding with his nanny in a black Lexus SUV earlier today when the car was involved in a "very minor fender bender" while driving in Coldwater Canyon. We're told Pete immediately rushed to the scene to check on his young son.
Pete's rep claims everyone involved in the accident was "fine."
Question is ...
The Senator skewed totally young this weekend, sharing the stage with Pete Wentz for a charity event. Inhofe's even dressing like the youngins ...he and Doogie Thompson sported matching cowboy boots.
Apparently unable to hock their baby pics to the highest or even lowest bidder, Ashlee Simpson and Pete Wentz posted pics of their bundle of joy online for free.
Wentz had blogged earlier this month they were offered "mounds and mounds of money" for the pics, but declined.
We contacted BMW, who told us: "A new born baby is a magical moment for any family and we are happy that their son shares the same initials as us, but as for a free car on his sixteenth birthday we can not make any promises."
Looks like he'll have to settle for a tour bus instead.
These two have more in common than just a flat iron ...
- Wannabe punk poser in hipster-lite boy bland
- A talentless media whore baby mama
This town ain't big enuf for the both of 'em!