The move comes almost exactly five years after Giffords, then an Arizona Democratic congresswoman, was shot in the head ago during an appearance in front of a Tucson supermarket. She barely survived and stepped down from her House seat to recover. She and her husband, a retired astronaut, have become outspoken advocates for tighter gun laws. The couple have founded a political action committee, Americans for Responsible Solutions, to push their new project.
"Time and time again, Hillary has done and said what is right, not what is politically expedient. That's why we are supporting her for President of the United States," the couple said in a statement to CNN.
The couple's endorsement -- which is tied almost exclusively to Clinton's position on gun control -- is a personal decision for the family, and does not mean their PAC, Americans for Responsible Solutions, will back Clinton.
"Most of the people running for president talk a lot about how tough they are. But most of them have shown they aren't tough enough to stand up to the gun lobbyists. Nobody knows better than us that talk is cheap," the couple says in their endorsement of Clinton.
Clinton has been outspoken about gun control during her run for president, including the issue in nearly every speech she delivers.
In the wake of a church shooting in Charleston the killed nine people, Clinton said it was time to "tackle" the issue of guns "with urgency and conviction."
"The stakes are too high, the costs are too dear, and I am not and will not be afraid to keep fighting for common sense reforms and along with you, achieve those on behalf of all who have been lost because of this senseless gun violence in this country," she said in June.
Clinton has also used guns as a political tool against Vermont Sen. Bernie Sanders, her top Democratic opponent. In an interview on Sunday, Clinton said that Sanders' position on guns was out of step with most Democrats and charged him with avoiding responsibility on the issue.
They argue in their statement that the gun lobby "has had a stranglehold on Washington" for years but that Clinton could break through that gridlock.
On Sunday Clinton welcomed the pair's endorsement.
"We need a president who will keep guns out of the hands of dangerous felons, not create loopholes that allow shooters like the one in Charleston to buy guns," she said in a statement. "We owe that to the countless families who have lost loved ones. We owe that to all America's children. We owe that to Gabby and Mark."