The 31-year-old Slovakian international, who spent four years at Tynecastle between 2008 and 2012, has penned a contract with Saints until January.
Kello will go into the Saints' squad for Saturday's trip to Dundee United, subject to international clearance.
On-loan Swansea keeper David Cornell started the season as first choice for the Paisley side, but lost his place to Frenchman Chris Dillo.
However, Dillo has been ruled out of the Tannadice trip through injury and Kello could make his Saints bow if selected by Buddies boss Danny Lennon.
The ex-Hearts number one arrived in Scotland following spells with Slovak side MFK Kosice, FC Vitkovice from the Czech Republic and Lithianian outfit FBK Kaunas.
After leaving Tynecastle following a row with then owner Vladimir Romanov, he was linked with a move to Rangers, but eventually signed on with Romanian side FC Astra Giurgiu before moving to Wolves on another short-term deal in March.