Jessica Alba and Cash Warren can officially close the book on their marriage ... because their divorce is now a done deal.

TMZ has obtained the docs confirming the former couple’s split has been finalized, bringing their nearly two-decade-long relationship to a legal end.

The two will share joint custody of all three kids -- who are 17, 14, and 7 -- neither will pay spousal support. A $3 million payment will go to Cash to even out the division of their assets.

We are told Jessica is paying that in 2 non-taxable installments ... $1.5M now, and $1.5M a year from now.

And … Jessica's legal last name is officially restored to Alba.

TMZ broke the story... Jessica filed for divorce in February last year, and sources told us at the time it was extremely amicable -- coming shortly after the two publicly announced their separation. Disso queen Laura Wasser represented Alba in the proceedings ... attorney Adam Lipsic repped Cash Warren.

Jessica has already moved on ... she’s been dating “Captain America” star Danny Ramirez for a couple of months now.

Cash has also been back on the dating scene, sparking rumors with actress Seanna Pereira and model Hana Sun Doerr.