'Gotham' Star Morena Baccarin -- I'm Marrying My Co-Star Ben McKenzie

"Gotham" star Morena Baccarin -- who is pregnant with co-star Ben McKenzie's baby -- plans to marry him ... TMZ has learned.

Baccarin is in the middle of a nasty divorce with director Austin Chick. According to legal docs obtained by TMZ, Baccarin says she got pregnant with Ben's baby in June, which she claims was after she separated with Chick. The estranged husband begs to differ and says she got pregnant while they were still together.

Chick filed for divorce, and Morena has already planned her next move. In her legal docs, obtained by TMZ, she says, "Today, I am in a new committed relationship. I am planning to re-marry. Also, I am 3 1/2 months pregnant."

Morena's divorce is bitter. As we reported, Chick says he came to their house in June, only to find Ben freshly showered and playing with their 1-year-old son.

And Morena included texts in her docs to show how bad the situation is ... she texted to Chick, "I'd like to be there alone with Julius [their son]. Please Friday all day and night through Sunday morning."

He responded, "I'd like to run Fox Studios and remake The Sound of Music w an entirely black cast of hip hop artist."