"We're excited and incredibly happy about our engagement and we look forward to a wonderful future together," the longtime couple announced in a joint statement.
Lachey, 36, popped the question to Minnillo, 29, on Thursday "near a beach" in California, with an Asscher-cut diamond flanked with trapezoids from Bader & Garrin, a source tells PEOPLE.
On Thursday afternoon, Lachey took to Twitter to write, "Hey all. If anyone's heard the rumors of my engagement, they are absolutely true! Vanessa and I couldn't be happier. Thanks for all the love."
"They are very happy," the source adds. "They make each other happy."
The couple, who split briefly in 2009, were not apart long before getting back together. "I am happy in a relationship," Lachey said at the time.
For her part, Minnillo hasn't been quiet about her desire to marry her longtime beau. "Once again it looks as if I'm always the bridesmaid but never the bride," she said, gently ribbing Lachey last year.
As for how they keep their relationship hot, Minnillo recently revealed that the couple shower together twice a day. "It's not a sexual thing and it's not a romantic thing," she said. "It just becomes an intimate thing ... I've had some of my most intimate conversations with him in the shower."
The duo began dating in 2006.
The will be the second marriage for Lachey, who was previously married to Jessica Simpson.