Here are 10 things you should know about him:
1) He's been a governor, a senator and a national party chair
Kaine has held three big-time political positions: Governor of Virginia from 2006 to 2010, DNC chair from 2009 to 2011, and US senator from 2013 to today.
And his political rise has been a methodical one, going from city council (1995-98) to mayor of Richmond (1998-2001) to lieutenant governor (2002-06) to governor. Prior to entering politics, he was a clerk in the US Court of Appeals for the Eleventh Circuit and then a lecturer at the University of Richmond law school.
2) He was a Christian missionary, and speaks Spanish
In the 1980s, he took a break from his studies at Harvard Law School to spend time as a Christian missionary in Honduras. While there, he taught carpentry and welding.
And in part thanks to his time in Honduras, Kaine is a fluent Spanish speaker. After winning his Senate seat in 2012, he became the first US senator to deliver a speech from the Senate floor in Spanish.
It's hard to overstate how big an asset each of these are for Kaine, politically. In the veepstakes, one of his opponents was Housing and Urban Development Secretary Julian Castro, who is of Cuban-American descent and grew up in Texas but doesn't speak Spanish. And for Democrats, having a Christian missionary on the ticket could help appeal to more religious voters who tend to lean republican.
"My faith is central to everything I do," Kaine told Patch. "My faith position is a Good Samaritan position of trying to watch out for the other person."
3) His eyebrow is/was a meme
Kaine's first big moment on the national stage was his Democratic response to George W. Bush's 2006 State of the Union address.
The appearance was perhaps best-remembered for this fresh-faced governor's constantly arching left eyebrow. It became a meme. "Tim Kaine Eyebrow Watch: Still Arched, Weeks Later," snarked Wonkette. Below, Cenk Uygur does a good job explaining the whole thing. (It was once even rumoured to hurt his chances at becoming Barack Obama's vice-president in 2008!)
4) He's known as a nice guy's nice guy
The headline of a recent Washington Post story about Kaine's potential VP prospects is titled, "What's a nice guy like Sen. Tim Kaine doing in a campaign like this?"
"The boy is cleaner than the board of health," Dave "Mudcat" Saunders, the eminently quotable Democratic political consultant told our Paul Schwartzmann. "If it's one thing Hillary needs, it's clean."
And Kaine's amiable style has served him well. His approval rating in polls has generally been significantly higher than his disapproval rating, and he was elected senator in 2012 by a six-point margin over Republican George Allen - a fellow former governor and former senator who was trying to reclaim his seat.
But Kaine's nice-guy demeanor has also raised questions about whether he would be able to play the traditional role of a running mate: attack dog.
5) He hasn't always been a Hillary backer
In fact, he was one of Barack Obama's earliest endorsers in the 2008 campaign, offering his support as early as February 2007.
Never one to wait around, his endorsement of Hillary Clinton in the 2016 campaign also came very early - in May 2014.
6) He was said to be favoured by Bill Clinton
As the New York Times's Amy Chozick and Jonathan Martin reported on Wednesday:
As Hillary Clinton prepares to make her choice for a vice-presidential candidate, Bill Clinton has privately expressed his support for Senator Tim Kaine of Virginia, according to three Democrats briefed on the conversations with the former president this week.
7) He married into a political family
His wife, Anne Holton, is the daughter of former Virginia governor A. Linwood Holton Jr. Holton, who met Kaine at Harvard, is the second daughter of a Virginia governor to later become the first lady of a Virginia governor. The first was Thomas Jefferson's daughter, Martina Jefferson Randolph.
Anne Holton is a former juvenile court judge who currently serves as the Virginia secretary of education, a position she was appointed to by Gov. Terry McAuliffe (D) in 2014.
8) He is a Catholic - and personally pro-life
But from a public policy standpoint, he has a pro-choice record. He explained it last month on "Meet the Press."
"I would say people use labels all the time, but I'm kind of a traditional Catholic: Personally I'm opposed to abortion, and personally I'm opposed to the death penalty," he said, adding: "I deeply believe - and not just as a matter of politics, but even as a matter of morality - that matters about reproduction and intimacy and relationships and contraception are in the personal realm. They're moral decisions for individuals to make for themselves. And the last thing we need is government intruding into those personal decisions."
9) He's musical
He likes jazz, sings and plays the harmonica.
His band is called the Jugbusters. (Warning: They rock pretty hard.)
10) He's no Bernie Sanders or Elizabeth Warren
Kaine launched his career in what was then a red state, and he's crafted a relatively moderate persona. He's certainly not the kind of pick that will get liberals excited in the way that Elizabeth Warren or Bernie Sanders - or even Senator Sherrod Brown, D-Ohio, - would have.
But Mother Jones wrote recently that, "Behind the Virginia senator's moderate reputation is a history of quiet progressive activism."