He is officially the 25th most followed person in the world, ahead of superstars like Beyoncé, Alicia Keyes, and Eminem.
The 21-year-old, who recently celebrated his landmark birthday in Las Vegas, tweeted his delight yesterday at reaching the milestone.
“Oi oi! 20 million followers! Amazing! Thanks everyone! Welcome to my world all the new followers,” he tweeted.
The Mullingar boyband member, who has just finished the Where We Are world tour, told his followers yesterday he was heading home to spend time with family and friends, but is already looking forward to going Down Under next spring for more stadium dates.
“Guys, last show done in Miami! thank you everyone for coming to see us this year! Love you all so much! See ya soon! Rest time now! Xxxx
“Our next show is in australia in feb! Can’t wait to get to oz! Best place on the planet”, he tweeted.
He added that he knows how lucky his band is to be able to play sellout gigs at stadiums around the world.
“Not many artists get to play stadiums, and you guys gave us that chance this year! I love you all! gona rest up for a while,” he tweeted.
As the 25th most popular person on Twitter, the Irish singer is just behind superstar footballer Kaka and Oscar-winning singer, Adele, and has almost twice as many followers as Kanye West and X Factor guru, Simon Cowell, who both have over 10m followers.
Horan has posted almost 10,000 tweets to his fans since he joined the site shortly after his band shot to fame on The X Factor four years ago.
Harry Styles and Niall Horan have both hit over 20m followers, while Louis Tomlinson and Liam Payne are hovering at 18.1m and 17.6m respectively. Engaged Zayn Malik has just over 13.3m followers as he briefly left the site last year and is still trying to get back on par with his bandmates.
But they are all trailing behind the queen of Twitter, Katy Perry, who has the most followers at 58m. Justin Bieber comes in second with 55m.
Female superstars dominate the top 10 most-followed on Twitter, with Taylor Swift, Lady Gaga, Rihanna, and Britney Spears all hitting over 37m.
Horan began his journey to superstardom when he auditioned in front of Simon Cowell, Louis Walsh, Katy Perry, and Cheryl Cole in Dublin in the summer of 2010 when he was just 16.
Before his audition, he told the cameras he wanted to be like Beyoncé and Justin Bieber and sell out arenas worldwide.
The nearest Irish star to the boyband superstar is Rory McIlroy with almost 2.2m followers and the chart-topping band The Script with over 2m.
Comedian Dara Ó Briain has also garnered a huge following on the social networking site with 1.8m, while chat show king Graham Norton has 850,000 followers.
Saturdays star Una Foden is the most followed Irish female star with 798,000 followers.
Horan’s Twitter supremacy has even rubbed off on his family, with his brother Greg Horan garnering almost 1.5m followers simply due to his sibling’s stardom.