Defender makes it two in a day for Joe.
Since then, he has had loan spells at Chesterfield and Rochdale, and the 21-year-old has made over 60 appearances in all competitions in his career to date including making the England U19s side.
The Glasgow side has signed the central defender after he impressed in his early days at Stockport County.
6'4" tall Josh signed for Portsmouth in the summer after agreeing to cancel his contract at Scottish giants Celtic.
"As I said with John Sullivan's signing earlier in the day, it's crucial we have competition for places in order that we can bring out the best in the players we have at the club."
"Josh is a player who I've known since he was at Stockport, and he has wonderful potential as a centre back," boss Joe Dunne told the official website.
The former Celtic centre half has joined the U's on a month's loan and, subject to paperwork clearance, will be part of the U's squad for the weekend game at the Alexandra Stadium.
Portsmouth defender Josh Thompson has become the second new player to join Colchester United ahead of their trip to Crewe on Saturday.