"She accepted with glee," a source tells Us. "She is madly in love."
Another source adds, "They were friends before, the groundwork had been laid before. They had always been attracted to each other, but always in another relationship. It may seem quick, but they knew it was right. They believe in the same things. They are dedicated to their work and helping others."
Us broke news in July that the Vampire Diaries hunk, 36, and the Twilight actress, 26, were dating. They moved in together three weeks later in Atlanta, Ga., and have since adopted horses Eagle and Millie, as well as a kitten named Sohalia. They're also parents to dogs Nietzsche and Enzo.
Somerhalder first sparked rumors of an engagement when he was spotted ring shopping at Neil Lane last month. "Ian bought a vintage diamond eternity band for Nikki," a source told Us at the time. "It's not a typical engagement ring but actually a wedding band you purchase for when you get married."
As if that weren't enough, Somerhalder hinted that 2015 was going to be a wonderful year for the couple in an Instagram message he posted on Jan. 8.
"How I spent my New Year's Day- in a #tbt way- wow. Week one of 2015," he captioned a selfie of the future spouses. "Thank you all for giving me so much strength-we're ready to kick major ass and make a great future starting now. Happy 1st week of the New Year- I hope you're on track with making your future the most sustainable and harmonious one possible…"
This will be the first marriage for Somerhalder and the second for Reed. The O.C. alum announced her split from American Idol season 10 contestant Paul McDonald in March 2014 after nearly three years of marriage. The exes, who amicably still make music together, finalized their divorce earlier this month.
Somerhalder previously dated his TVD costar Nina Dobrev for three years until their split in May 2013. In an odd twist, Dobrev dated Dancing With the Stars pro Derek Hough in September 2013, before he briefly moved on to Reed in April 2014.
Sign up now for the Us Weekly newsletter to get breaking celebrity news, hot pics and more delivered straight to your inbox!