The department announced the charges against Det. Sgt. Michael Erickson on Tuesday.
Chief David Splain said in a statement that the charges were brought by the Pennsylvania Attorney General's Office.
Splain went on to say his department had no role in the investigation.
The charge stemmed from an incident in Aston Twp. while Erickson was off-duty.
A spokesperson for the Pennsylvania Attorney General's Office says the alleged assault involved an unconscious person on June 1.
The department was notified about an investigation that began in the Delaware County District Attorney's Office, and Erickson was immediately placed on administrative leave, Splain said.
The case was turned over to the Pa. Attorney General's Office on July 2.
Erickson resigned on Monday and surrendered to authorities on Tuesday.
Erickson first became an officer with the Nether Providence Police Department in 2003.
He started as a patrol officer and worked his way up to detective sergeant.
Splain said the department is taking the investigation "very seriously" and it is cooperating fully with investigators from the Pennsylvania Attorney General's Office.
Erickson's bail was set at $200,000. A preliminary hearing is set for Oct. 9.