Officers tracked the device in the bag to a Culver City bus traveling southbound on Sepulveda Blvd. just north of Palms Blvd.
Police stopped all traffic on Sepulveda and had passengers exit the bus individually.
Police found the suspect, described as a 60-year-old man, with a bag with money and a gun inside. The suspect was taken into custody.
The suspect allegedly robbed the Bank of America branch on the 10900 block of Wilshire Blvd. at about 4:55 p.m. Thursday.