Mount Pleasant police are investigating the Monday robbery of the Heritage Trust Federal Credit Union at 112 Johnnie Dodds Blvd.
A man enter the credit union at about 3 p.m. and jumped over the teller station. He left with an undetermined amount of cash taken from the drawers.
He is not thought to have brandished a weapon, police said.
He was described as possibly a white male in his 20s, up to 175 pounds in weight and between 5 feet, 7 inches and 6 feet tall.
He was wearing all dark clothing, gloves, a stocking cap and a bandanna over his face. Anyone with information can call Mount Pleasant Police at 884-4176.
Meanwhile, Charleston County sheriff's deputies have arrested a second suspect in the May 7 robbery at a Seabrook Island bank.
Quentin Anthony Gibbs, 24, of Charleston was charged with bank robbery after turning himself in on May 28, Maj. John Clark said.
Gibbs is the second suspect to be arrested in the case. Last month deputies arrested Charlton Dwayne Smith, 21, of Johns Island.
The men are accused of robbing the BB&T branch at 1900 Seabrook Island Parkway.
Clark said they still are investigating whether the men were involved in a robbery at the same bank on April 14. The two robberies appear to be similar, he said.
Affidavits state that a witness contacted deputies and identified Smith and Gibbs as the men who robbed the bank.