The 34-year-old singer, reportedly took the permission from Turchin's parents before proposing him with a black diamond band, reported Hollywood Life.
Bass took to micro-blogging site Twitter to share the news with the fans.
"He said Yes!! Love this man," Lance tweeted along with a picture of his boyfriend showing off his ring.
"They're such a great couple, everyone is so happy for them. Their friends and family are thrilled," a source said.
NSYNC, an American boyband consisted of Justin Timberlake, JC Chasez, Chris Kirkpatrick, Joey Fatone, and Bass remained active till around 2002. The band recently reunited to perform on-stage after a decade during MTV Music Video Awards.