The aw-shucks news was confirmed Friday by a publicist for Simpson's Columbia Records.
No word on how the popster popped the question, and no wedding date has been set.
The photogenic duo--perhaps the coziest Tiger Beat twosome after Justin and Britney--have been harmonizing since hooking up during a 1999 tour. It didn't take long for them to get disgustingly cozy, as evidenced by the following exchange during a joint interview on CBS' Early Show two years ago:
"The first time I ever heard her sing, I got chills," Lachey said. "And I told the guys..., 'I'm going to marry that girl one day.' "
"We definitely knew--no ifs, ands or buts--we're totally in love," added Simpson.
"You know," said Lachey, "it sounds kind of hokey and cheesy, but at the same time, it's cool."
So cool, that Lachey, now 28, has remained faithful to his 21-year-old fiancée despite her public proclamation that she's holding onto her virginity until she gets married.
The two became a fixture on MTV with their hit duet, "Where You Are," off Simpson's multiplatinum 1999 album, Sweet Kisses. "Nick and I are going to be the next Sonny and Cher," Simpson once announced to Billboard. "Only, I expect we're going to last a lot longer."
Lachey also penned a couple of songs, including "Forever in Your Eyes," for Simpson's follow-up, Irresistible
Last February, the engagement rumors went into overdrive when Simpson was spotted sporting a gold-and-diamond ring that she said was "more than a friendship ring."
But things weren't as hunky-dory as the duo made it seem. The treacle momentarily stopped oozing last spring when Simpson broke the news that she and Lachey--frequently separated by touring and other pop-star commitments--were splitting. "Right now, my main focus is my career," Simpson told Twist magazine. "I pray that each of you support this decision and trust us in God's hands."
God was apparently looking out for them, because they recoupled soon after and reportedly bought condos (separate, of course) in a swanky Los Angeles highrise.
Simpson and Lachey have plenty in common. Both come from very select gene pools and both toil in the shadows of better-known peers--Simpson ranks behind Spears and Christina Aguilera in the teenybopper pantheon and Lachey's 98 Degrees has never generated as much heat as 'N Sync or the Backstreet Boys.
Unlike Britney and Christina, Jessica failed to make the Mickey Mouse Club. Instead, she spent her pubescent years on the Christian music circuit, before being signed by Columbia and crossing over with Sweet Kisses.
Lachey formed 98 Degrees with his brother, Drew, Jeff Timmons and Justin Jeffre. "Invisible Man," the first single from their self-titled debut album, was a top 20 hit in 1997. The group's sophomore disc, 98 Degrees and Rising, was issued in 1998, followed a year later by the Yule-themed This Christmas. Their latest album, Revelation, was released in 2000.
According to the official 98 Degrees Website, the group is currently "on an extended break [and] have no immediate plans to record or tour." And, unlike the recently betrothed Nick and Jessica, 98 Degrees' "future [is] unclear at this point."