The 32-year-old, who has over 350 appearances since coming through the Lions' ranks, will move to Fratton Park for the first month of the club's League Two campaign.
Pompey manager Andy Awford told the club's official website: "He's an experienced Championship centre-back - and that's an area where we're a little bit light on numbers.
"I'm looking forward to working with Paul and we're grateful to Millwall for loaning him to us."
Robinson, who scored the decisive goal in the play-off final to send Millwall to the second tier in 2010, told Millwall's website: "It's a good chance for me to go and get some games under my belt.
"Portsmouth are a massive club and the chance to play so many games in 28 days was too big an opportunity to resist.
"Hopefully I can go there to improve my match sharpness and help a big club on its path to getting back to where it belongs."