The winger becomes the Blues’ second signing of the evening - and eighth this summer after impressing while on trial.
The 20-year-old has signed a one-year deal, with Pompey having the option to extend that agreement by a further 12 months.
Reid caught the eye of supporters when he replaced Ronan Curtis in the pre-season win over Hawks.
The former Manchester United academy player has continued to make a positive impression on Danny Cowley, prompting the Pompey boss to make his move.
Reid arrives after leaving Birmingham this summer and has also played academy football for Swansea City, as well as turning out for Barrow and Walsall.
Cowley explained he is sees Reid as a promising talent with the ability to grow at Fratton Park.
He told Pompey’s official site: ‘Jayden is an exciting young player with a lot of potential. He’s a good dribbler and has pace, athleticism and energy.
‘Also, which is important for us, he has a love for the game and it’s been enjoyable working work with him over the past week or so.
‘We’re looking to put a group of talented young players together that have real potential – and Jayden joins Haji Mnoga and Liam Vincent in that bracket.
‘There’s a lot to work with, but also a lot to work on. The main thing for us is that we can see has the attitude of wanting to learn and develop.
‘The young players have an infectious enthusiasm and they drive up the standards in training every single day.
‘We want that balance of youth and experience to create an environment where everyone’s improving all the time.’