Sanford confirmed reports he proposed to Chapur in a Buenos Aires restaurant, The (Charleston, S.C.) Post and Courier reported Sunday.
He told the newspaper he released a statement to the media confirming the engagement and that his "closest friends have met and love her, and I look forward to introducing her to still many more that have yet to do so."
Sanford, South Carolina's governor from 2003-11, had been considered a potential Republican presidential candidate until July 2009, when a scandal involving Chapur ended his political aspirations.
After being unreachable for about a week, his staff said Sanford was hiking the Appalachian Trail. He later admitted he was in Argentina to be with Chapur, with whom he'd been having an affair.
Sanford and his wife, Jenny, divorced in 2010. They have four sons.
Sanford faced more than three dozen ethics charges over disputed use of state aircraft. The charges were dismissed by a legislative panel, and the full Legislature voted to censure him for traveling to Argentina to meet Chapur.