At a rally in Miami, Mrs Clinton reeled off the Virginia senator's career achievements, including his work decades ago as a civil rights lawyer and time spent with missionaries in Central America, to depict him as a man dedicated to social justice.
"Senator Tim Kaine is everything Donald Trump and Mike Pence are not," Mrs Clinton said to a roar of approval from the crowd.
"He is qualified to step into this job and lead on day one," she added, praising his work as governor of the key swing state of Virginia and his record of shepherding it through hard financial times.