Bledel's representative confirmed the happy news to UsMagazine.com Wednesday.
"Vincent proposed a few weeks ago. She's been wearing her ring. It's huge," an insider told the celebrity news website.
"They couldn't be happier," a second source said.
Bledel, 31, is best known for her work on TV's "Gilmore Girls."
Kartheiser, 33, is a cast member of the drama series "Mad Men."