Under Armour now has three NFL quarterbacks in its endorser backfield.

Cowboys quarterback Tony Romo signed with the company on Thursday after two years of being a free agent — and wearing Nike cleats with paint-covered logos.

Details were not disclosed.

“Tony Romo is one of the most respected and accomplished athletes in the NFL,” Under Armour said in a statement.

The 35-year-old joins the Patriots’ Tom Brady and the Panthers’ Cam Newton in promoting the brand.

Under Armour’s large group of sports stars is a good strategy because they haven’t gone overboard or paid them too high, FBR analyst Susan Anderson said.

“They continue to look for ones that make sense,” Anderson told The Post.

Under Armour shares fell 2.8 percent on Thursday, to $98.23.