After detailing his previous adventure trips, "profound frustrations" and "emotional" exhaustion following the recent legislative session, he eventually acknowledged having an extramarital affair.
"So, the bottom line is this: I have been unfaithful to my wife," Sanford, 49, told a crush of reporters.
Sanford's admission comes in the wake of a similar revelation from another rising Republican star -- Nevada Senator John Ensign, who last Tuesday admitted cheating on his wife but did not resign his Senate post.
Sanford, once considered a potential Republican contender for the presidential nomination in 2012, said he would resign from his position as head of the Republican Governor's Association, but refused to say whether he would give up his post as governor.
First elected in 2002, Sanford has one year left in his second term as governor, and is constitutionally prohibited from running for another term.
The press conference Wednesday was called after days of media speculation as to where Sanford spent a period of five to seven days during which his staff, and even his wife, were unable to confirm his whereabouts.
Sanford's staff at first said the governor was hiking the Appalachian Trail, but then retracted that statement, provoking a media frenzy that culminated in Wednesday's bizarre press conference.
The governor spent much of his opening remarks offering apologies -- to his wife and four children, his friends, supporters and voters from South Carolina -- before announcing his adultery.
Sanford then admitted that he had spent a week with an Argentine woman with whom he had conducted a five-month affair that developed out of an initially "very innocent" eight-year friendship conducted mostly via email.
"Here, recently, over this last year, it developed into something much more than that," Sanford said.
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