The 24-year-old openside was chosen for the post ahead of Ireland's Brian O'Driscoll and Paul O'Connell by head coach Warren Gatland, with whom he has forged a close relationship with Wales.
"Warren told me a week ago Sunday. For the last nine or 10 days it's been the hardest secret I've ever had to keep," he said. "You just want to tell people, but I managed to keep it to myself. It's unbelievable - an accolade that very few people are able to achieve. To have the opportunity to do this is an unbelievable honour."
He added: "Obviously it's going to be new for me and I'm going to learn along the way. I can't sit here and say I know what's going to happen because I don't.
"There's a good Welsh contingent of leaders who I can rely on and there are other experienced players in the squad who can help me along the way.
"It's an experience I really can't wait to begin. I'm really looking forward to it now and will just keep doing what I do for Wales because I guess that's what Warren has picked me for."
Gatland explained his thinking behind choosing Warburton, who heads a 15-strong Welsh contingent with 10 English, nine Irish and three Scots completing the touring party.
"In the last two or three years he's been the most successful captain in the northern hemisphere," Gatland said.
"When we sat down to pick a squad - and it's a fantastically strong squad - we decided that Sam would be one of the names among the starting Test XV.
"I have a huge amount of respect for Sam. He's an absolute professional who leads from the front and I believe he'll do a fantastic job."
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- Warren Gatland