The Scotland international played the first half of the season at Scunthorpe United, but was released from his contract at Glanford Park earlier this month.
Iwelumo has made 14 appearances this season, scoring two goals, and his new manager Tommy Wright hopes the 35-year-old can help keep St Johnstone in the top six before the league splits in April.
"I'm delighted to get someone with his experience," he said.
"He'll bring a lot on and off the park, he has played at a good level and has the knack of scoring goals.
"Also, importantly, he has a good physical presence about him which we are looking for. I'm delighted to get him."
The former Wolves and Charlton Athletic striker has four Scotland caps, his last coming in October 2010 against the Czech Republic.
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