Decoy operations helped reduce the number of robberies in the subway system by 22% last year, the NYPD's top transit cop told the MTA board Monday.

Electronic devices were a popular target for subway muggers and pick pocketers, making up 54% of stolen property in all robberies and grand larcenies.

"Clearly, our decoy operations were critical to reducing these electronic thefts," Joe Fox said, adding the NYPD will "continue to use decoy operations frequently as part of our broader strategy to reduce theft of electronic devices."

In recent years, transit police had beefed up its presence in the subway system as pricey smartphones became the top target for criminals.

During 2013, transit police hatched 248 decoy operations, in which plainclothes officers present would-be thieves an opportunity to grab a device from a seemingly unsuspecting rider.

Those operations last year netted 96 arrests, with 70% of those caught in the act having a criminal history. Nearly 40% of those arrested with a criminal history committed a crime inside the transit system. The total number of arrests in the system last year went up 6.7% to 51,368 -- a difference of 3,228 arrests.

While robberies were down, grand larcenies were relatively flat, trickling up 3%, Fox said.

Altogether, there were 2,606 major felonies committed in the transit system last year -- 126 fewer than in 2012, a 4.6% drop.

The reduction in crime coincided with record ridership. Last year, the system averaged 7.1 felonies a day, from 7.5 felonies in 2012 -- a statistic Fox said was an "encouraging result" because of the ballooning number of riders.

Eugene O'Donnell, a former NYPD officer and professor at John Jay College of Criminal Justice, said the drop in crime has many riders showing "a real comfort level" in using electronic devices on the subway.

"You see a lot of people using these devices like they're in their living room at home," O'Donnell said.

He called the number of stings "modest," but added that they are a deterrent to crime, sending a message that makes potential robbers think twice before stealing a smartphone.

"You don't necessarily need a lot of it to be effective with the criminal population," O'Donnell said. "Word gets out."