RELATED: What you need to know about Tim Kaine
In the message, Clinton says: "I'm thrilled to tell you this first: I've chosen Sen. Tim Kaine as my running mate."
Kaine is expected to join Clinton at a rally in Florida on Saturday.
Just got off the phone with Hillary. I’m honored to be her running mate. Can’t wait to hit the trail tomorrow in Miami!— Senator Tim Kaine (@timkaine) July 23, 2016
A favorite since the start of Clinton's search for a running mate, Kaine is a former governor of the battleground state and former mayor of Richmond.
Kaine won election to the Senate four years ago after serving as the chairman of the Democratic National Committee.
Clinton is choosing Kaine from a group that included Agriculture Secretary Tom Vilsack and Massachusetts Sen. Elizabeth Warren.