Ryan Phillippe's ex/alleged baby mama has lawyered up ... and a legal battle is now brewing ... TMZ has learned.

Alexis Knapp -- the model/actress who claims to be carrying Ryan's baby -- has hired Mark Vincent Kaplan.

Ryan has said he'll take a DNA test once the baby is born and support the child if it's his, but Alexis is not satisfied.

And here's a fun fact. Kaplan repped K-Fed in his divorce/custody battle with Britney Spears.

Credit: TMZ