Burnley secured automatic promotion back to the Premier League in early April after only one season in the second tier, finishing first in the Championship.
The couple said they look forward to strengthening Burnley's international presence as well as contributing to the community.
"When you invest in a club that’s been around since 1882, you must have great respect for its history and tradition," they said in a statement. "
We understand that not only are we investing in the squad and manager, we’re investing in the town and its people."
Chairman Alan Pace said the couple would add tremendous sporting success and pedigree to the club and help bring in an international audience.
"This is a conversation that has been happening for some time and over the course of this season we’ve been hugely impressed by their passion and interest in both the club and the Burnley community."
Financial details of the investment were not disclosed.
Watt, three-time Defensive Player of the Year in the National Football League, announced his retirement in December. He is widely considered among the greatest-ever defensive players.
Kealia Watt formerly played for the Chicago Red Stars of the National Women's Soccer League and is currently a free agent.
Burnley are owned by Burnley FC Holdings Limited, with Calder Vale Holdings holding 83.97% of the company and individual shareholders holding the remaining shares.
