Lewandowski, who joined from Borussia Dortmund in 2014 on a free transfer, has established himself as one of the best strikers in Europe.
His average scoring rate of a goal every 1.5 games at Bayern has seen him attract plenty of attention from other clubs.
Amid reported interest in Lewandowski from the Premier League, Beckenbauer has opened up speculation about a move to La Liga under Zinedine Zidane.
"Robert Lewandowski has been mentioned as a possible candidate for Real Madrid," Beckenbauer told DPA.
However, Beckenbauer has squashed any potential rumours about Thomas Muller, insisting his future is with the reigning Bundesliga champions.
"[Muller] has assured he wants to continue at Bayern, which will make the fans want him even more," he said.
Bayern are on track for a third successive Bundesliga crown, holding an eight-point gap over Dortmund in the table.