The famous country music couple -- who've been together for over a decade now -- are ending their marriage of 11 years ... this after Jason ran to court in Tennessee this week and filed for divorce, citing irreconcilable differences.

According to the docs, obtained by TMZ, Jason says their date of separation was Dec. 3 of 2023 ... and he goes on request joint custody of their 8-year-old daughter, Mercy.

JI also says he and Amanda will enter into a Parenting Plan ... and even submitted a certificate highlighting the fact that he's recently obtained a parenting skills certificate that helps people navigate their children through divorce. So, he's ready for the challenge.

No mention of spousal support here, but presumably ... all of that is hashed out in their prenuptial agreement, which he says was executed 2 days before their wedding in 2013.

One other thing ... there's a restraining order that was submitted in his filing -- but it's a standard thing there in TN, and has nothing to do with claims of misconduct. It's simply there to tell both parties not to move any assets until a judge can go through and divvy things up.

Jason says that while a prenup is in place, he notes there's separate property between him and Amanda that needs to be allocated and divided -- which will be handled later.

Like we said, Jason and Amanda have been together for a long time -- and the fact they often perform together is hitting their fans even harder now that they're splitting up.