Paul Robinson was popular with the Fratton faithful during his single season as a Pompey player.
And he hopes to keep them onside by helping shore up the Blues back-line in his new role as a part-time coach.
The 36-year-old defender will combine his duties with turning out for National League outfit Havant & Waterlooville.
And Robinson is thrilled with the opportunity to link up with his former Millwall manager Kenny Jackett once again.
He said: “It’s lovely to be back. I loved my year here as a player and this is a fantastic opportunity for me.
“I left a bit too soon for my liking, although that’s the way football is. I’m here again in a different role and it’s great to be at a club like Pompey.
"I’ve got an incredible amount to learn, but I couldn’t think of a better place to do that."
“We’re going in the right direction and ready to take off again with the new owners. It’s nice to be part of that momentum.
“I’ll be helping the defenders out and any other little extras that they might need. I’ll try to impart my knowledge from over the years to improve the goals against column.
“That will probably be my main role here and I’ll be in a couple of days each week because I’m only training part-time.
“I’m used to getting up in the morning and coming in to work, while I know that my playing career is reaching an end.
“It’s a big transition period and I’ll be honest – I haven’t done a load of coaching throughout my career, so it’s a totally different kettle of fish.
“I’ve got an incredible amount to learn, but I couldn’t think of a better place to do that and a better person to learn from. I’m grateful to Kenny for giving me the opportunity.”