The Blues tonight sealed their third signing of a hectic week of transfer activity with the midfielder’s arrival.
Tunnicliffe arrives on a two-year deal with Pompey holding an option to extend that agreement by 12 months.
Danny Cowley has beaten off a number of Championship clubs to the 28-year-old’s signature.
The likes of Derby County Huddersfield, Barnsley and Hull City were all said to be be keen on the player who becomes a free agent after leaving Luton Town.