The 23-year-old will sign a three-year contract with the Fratton Park team once the Royals have thrashed out a compensation deal with Portsmouth. He turned down a new contract at Reading as Steve Coppell refused to promise him Marcus Hahnemann's number one jersey.
Once Ashdown has signed for the South Coast side he will challenge 35-year-old ex-Reading goalkeeper Shaka Hislop for the club's number one shirt.
Despite turning his back on the club he supported as a boy, the emerging young goalkeeper relishes the idea of playing Premiership football.
"It's just too good an opportunity for me to turn down," explained Ashdown speaking to the Reading Evening Post.
With age beginning to get the better of Hislop, he will hope to make the first team spot his own at Fratton Park.
"It is every boy's dream to play in the Premiership. I'm still young and I have time to prove myself. I expect to start on the bench for Portsmouth, but I am hungry and I am up for the challenge," he added.
"Whenever an opportunity to go to the Premiership is offered, you have to jump at it and take it."
Steve Coppell called on Ashdown at the back end of last season to cover for injured first choice star Marcus Hahnemann. Young Jamie excelled in the role and would have surely held the number one shirt next season at the USA international's expense.
That Portsmouth made him an offer is testament to how well the ex-Rushden and Bournemouth loanee played during his ten game stint in the team.
Even though Ashdown is out of contract and technically a free agent, the South Coast side must pay a fee for the youngster as he is under 24. The Royals are believed to be holding out for £250,000 for the shotstopper who made sixteen appearances in Reading's first team.