The robbery happened just after 8:00 p.m. Friday at the Tourneau Jewelry store in the Plaza near Lord and Taylors.
Police say five to six men entered the store and smashed a display case, grabbing 29 high-end watches worth $688,000.
Chopper 6 was overhead as shoppers and employees evacuated the mall.
There were no reports of injuries.
Police believe the same crew pulled off a robbery at this same store In July, 2011. In that case, the thieves stole more than $576,000 in watches.
That means the two robberies have netted the bandits more than $1.2 million in watches.
Investigators also told Action News this gang may have pulled off 15 similar jobs in the area.
Officials say the common thread is that the suspects work fast, smashing glass cases and target expensive watches.
Upper Merion Police ask that anyone with information on the robbery call them at 610-265-3232.