The X Factor star tweeted a picture of her big day, telling her followers: "So excited, I couldn't help it. Candid friend shot… I'm still blushing."
Katie looks overjoyed in the black-and-white pic, radiant in a mermaid-style Vera Wang dress as she holds her handsome new husband's hand.
The 24-year-old's hair is gathered into a bun atop her head, and she wears a veil at the back of her hair.
She wrote later: "I have never smiled so much in my life! So proud, so happy, so in love, so excited!"
The newlyweds, who got together six months ago, are believed to have wed in New Orleans.
Katie, who found fame on Simon Cowell's TV star search in 2010, has been based in the US recently while she works on her music.
She announced her engagement to Brad in July, tweeting pictures of matching heart tattoos the pair got inked on their engagement fingers.
Brad – who used to be in the military before he took up modelling – later presented his wife-to-be with a sparkling diamond ring from Tiffany and Co.