The plaque was placed in memory of Ofc. Charles Knox, who was killed in the line of duty during the summer of 1992.
It was installed on a sidewalk location on the 2000 block of South Broad Street in South Philadelphia. An officer noticed the plaque was missing while on patrol Tuesday night.
The plaque was placed in 2008.
It's not known when the plaque was removed. Investigators say the amount of dirt and debris in the area means it's possible the plaque has been missing for some time.
Investigators checked with the Fraternal Order of Police and management at the nearby Walgreens store, but they haven't been able to find anyone who gave permission for the plaque to be moved.
Police are now reviewing footage from the Real-Time Crime Center cameras in the area for any possible clues.
"The Philadelphia Police Department is committed to preserving the memory of our fallen officers," Police Commissioner Kevin J. Bethel said in a statement. "We are taking this matter very seriously and are asking anyone with information to please come forward."
FOP Lodge 5 said Wednesday it is offering a $5,000 reward for information that leads to an arrest.
"I'm saddened and angered that someone would allegedly steal a missing memorial plaque dedicated to one of our beloved heroes, Officer Charles Knox. Our thoughts and prayers are with the Knox family on this sad day," FOP Lodge 5 President Roosevelt Poplar said in a statement.
Police say they just want the plaque returned, no questions asked.
"A lot of us use these plaques to remember what occurred, and it angers you when you see something like this happen," said Deputy Commissioner Frank Vanore. "So we're really urging someone to let us know."
If you can help in the investigation, you are urged to contact the South Detectives Division at 215-686-3013. You can also call or text the police tipline at 215-686-TIPS (8477).
Officer shot during armed robbery
Ofc. Knox was shot around 8:30 p.m. on Aug. 30, 1992, while responding to an armed robbery at a Roy Rogers restaurant in the 1400 block of Snyder Avenue.
Knox entered the restaurant and saw a suspect holding a gun to the manager's head. He ordered the suspect to drop his gun, but instead the suspect lunged at him.
During the struggle, police say a second suspect came out of the manager's office behind Knox, put a gun beneath his vest, and fired.
Knox fell, and the suspect shot him again while he was on the ground.
Both suspects were sentenced to life in prison for Officer Knox's murder.
He had served the Philadelphia Police Department for more than two years.
The nearby Walgreens stands where that Roy Rogers once operated. A memorial on the wall of the Walgreens will be removed by police both for its protection and because the Walgreens was shut down on Tuesday.