Manager Arsene Wenger confirmed that the 41-year-old, who left the club in 2008, had returned as cover after injuries to Lukasz Fabianski (shoulder) and Wojciech Szczesny (finger) left Manuel Almunia as the only senior goalkeeper available.
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"He has signed for us for the rest of the season," Wenger told a news conference on Thursday.
"I'm happy because when Jens is in training he is as motivated as ever. If he has to play, if I choose to play him, his huge motivational level will be vitally important."
Former Germany international Lehmann spent five years in north London and was ever-present when Arsenal won the Premier League in 2003-04 without losing a match.
He announced his retirement last year after a spell with Stuttgart but has been training with the Gunners this month.
OTHER CONCERNS
Wenger has other concerns as his side prepare for the final 10 games of the Premier League season, confirming that defender Thomas Vermaelen is out for the rest of the campaign with an Achilles injury.
"His season is over," Wenger said. "In fairness I never expected him to be fit before the end of the season."
Johann Djourou, a handy replacement for Vermaelen in the heart of Arsenal's defence, is also out for six weeks, according to Wenger, after dislocating his shoulder in the FA Cup defeat by Manchester United on Saturday.
Captain Cesc Fabregas and Theo Walcott will also miss Saturday's Premier league trip to West Bromwich Albion.
Fabregas has a hamstring injury and winger Walcott is still troubled by an ankle injury sustained last month and which will also rule him out of England's Euro 2012 qualifier against Wales in Cardiff next week.
Arsenal are three points behind Manchester United with a game in hand and still have to play Sir Alex Ferguson's side at Emirates Stadium.
