Reed started dating 'The Vampire Diaries' actor shortly after she split from her husband and singer Paul MCDonald, early last year, reported People magazine.
"She accepted with glee. 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," a source said.
The actor, 36, and Reed, 26, moved in together three weeks later after they confirmed their relationship in July and have since adopted horses Eagle and Millie, as well as a kitten named Sohalia. They're also parents to dogs Nietzsche and Enzo.
This will be the first marriage for Somerhalder and the second for Reed.