The goalkeeper, 23, follows Birmingham striker Jake Jervis in signing on the emergency loan deadline day at Fratton Park, and will replace Mikkel Andersen after he returned to Reading following the completion of his 93-day emergency loan deal.
Caretaker Blues boss Guy Whittingham told the club's official website: "Alex is an experienced keeper who will talk to the back four. He's someone that the defence can have a lot of confidence in.
"It's a big advantage that he's played plenty of games in this league and knows what it takes to do well in it."
Polish born Cisak has made six appearances for the Boundary Park outfit this season, keeping one clean sheet, but had been frozen out by manager Paul Dickov in recent weeks.
Jervis, 21, moves to Fratton Park having only recently concluded a similar spell at nPower League One leaders Tranmere, where he scored once in five appearances. He is eligible to make his Portsmouth debut at Coventry on Saturday.
- Sports & Recreation
- Mikkel Andersen
- Fratton Park
- Jake Jervis