The 18-year-old, who played at the World Cup for Australia, becomes Hull’s ninth addition of the summer so far, signing a five-year deal.
Harrington has had an incredible rise to prominence, only joining MLS side Colorado Rapids from Brisbane Roar in January.
Although Hull haven’t disclosed the fee, it’s been reported that the deal was completed for around £16 million, including bonuses.
Speaking to the club’s website, Herrington said: “I’m buzzing to be here. It’s a great project and a great club to be a part of. I’m so excited to be here.
“The opportunity to play in the Premier League, one of the best leagues in the world, is a dream come true. I’ve woken up at all hours in the morning in Australia to watch these games. To be a part of it is so special.
“To hear from the club and coaching staff has been so exciting for me. I can’t wait to meet all the boys and the fans. I’m here to work hard and hopefully get a lot of games in the Premier League.
“I spoke to ‘Jacko’ (Jackson Irvine), and he said very good things about the club and the area. He loved his time here. To hear that from him, after being with him at the World Cup, gave me a sense of calmness.
“It was a dream come true to represent my country at the World Cup – something I had always wanted to do. To achieve it at 18 was so special and to have my family in the stands meant everything to me.
“I’m really looking forward to the first game against Manchester United. I can’t wait to get started.”