A well-dressed bank thief desperately tried to quiet down a freaked-out Midtown teller — who screamed when he slipped her a note saying he’d kill people unless she handed over cash.

The Bank of America worker — described as a “sweet girl in her 20s and not originally from New York” — began yelling, “Oh, my God!” when she read the letter, sources said.

The dumbfounded crook then raised his index finger to his puckered lips and actually said, “Shhhhh.”

The note was scrawled in black magic marker and warned, “I have a gun and will use it. You have 5 seconds to hand over 100s, 50s, 20s before people start getting hurt.”

After screaming, the teller bolted to the opposite end of the Third Avenue bank and dived under the counter, sources said.

“She was legitimately scared,” one law-enforcement source said about the incident, which happened Wednesday at 10:10 a.m. “The guy caught her off guard, and she freaked out.”

The stunned 5-foot-10, 180-pound suspect just stood in silence for a few seconds — then bolted out, heading south on Third Avenue toward East 54th Street.

The bank was nearly empty. Only a manager and another teller counting bills were inside, sources said.

The manager called 911.

Responding officers interviewed the teller, who provided a detailed description of the dapper bandit, sources said.

Cops conducted a sweep of the area but couldn’t find the suspect.

He didn’t get any money.

The NYPD released a photo of him (above) and urged anyone with information to call CrimeStoppers at (800) 577-TIPS. All calls will be kept confidential.