Christian, 44, and business development consultant Brittany celebrated their engagement with a big party with family and friends earlier in the year, and had originally planned to wed in a small ceremony at a later date.
But as they arrived at the courthouse to get their marriage license, they spontaneously decided to get married there and then, People confirms.
Christian is currently splitting his time between Florida — where the couple live, and where Brittany works — and Illinois, where the Nymphomanic star is shooting an upcoming ABC show.
"I live in Miami now and, of course, (Mind Games) shoots in Chicago, which is very cold, so now that my blood has appropriately thinned out I'm sure this will run for eight years," Christian joked. "Yeah, that will be the irony of it."
The True Romance actor was previously married to Ryan Haddon. They have two children together, Jaden and Eliana.