"We just got engaged!" Duggar, 19, told Us. "I'm so excited to be engaged to my best friend -- Austin made it so romantic, it's amazing."
Forsyth, a longtime family friend, said he's excited about the big events to come in his relationship.
"I'm just overwhelmed with emotion," he told the magazine. "I'm so excited to be engaged, because that means the next step is marriage, and I'm excited to see what the future holds for us!"
Duggar and Forsyth have known each other for about 15 years. The couple announced in November 2016 that they had begun a courtship.
Forsyth chose a romantic spot in a wooded area on a hill to enter the courtship, which for the Duggar family, means supervised dates and no kissing or even hand-holding. The typical courtship leads toward an engagement.
"It's a place that I've come many times to just get away and spend time with the Lord," he said at that time of the special location. "I just knew that this was the place that I wanted to ask her."
The February 27 episode of Counting On featured Forsyth asking Duggar's father, Jim Bob, for permission to court his daughter.
"Over the years, I've been watching, and I really admire her heart for the Lord and her character," Forsyth told Jim Bob, according to Us. "She's such a diligent worker. She gets all the younger siblings working. She's a good delegator -- and, of course, her beauty."
Jim Bob was "honored" to give Forsyth his blessing, calling the young man a "good guy."
A little over a week before Duggar and her beau began their romance, her older sister Jinger married former soccer player Jeremy Vuolo in an Arkansas ceremony on November 5.
Counting On aired its Season 2 finale February 27 on TLC.