The American custodian had a successful loan spell at Rochdale earlier in the season, but has now stepped up a division in a bid to gain further experience.
Royals boss Alan Pardew (pictured) approached The Cottagers following an injury to first choice keeper Phil Whitehead.
Fulham goalkeeping coach Gerry Peyton believes it will be a beneficial move for Hahnemann.
"It's a very good move for Marcus," Peyton told Fulham`s official website. "Reading are a very ambitious club, with a new stadium and an ambitious chairman.
"Marcus has been to Rochdale and did exceptionally well there, this is now a league higher for him.
"I'm looking for him to get a good standard of training and match preparation at Reading."
