While responding to a call in February, the department said Officer First Class Joshua J. Hill "came into possession of jewelry belonging to an individual who was taken to a local hospital for treatment."
After keeping the jewelry for "some time," Hill, 34, later sold it and kept the money, police said.
A friend of the victim called police earlier this month after attempting to get the jewelry back.
"We are deeply saddened and embarrassed by the actions of this officer," Col. Thierry Dupuis said in a statement. "We want members of this community to know that we deal with matters such as this head on. We hold ourselves accountable and will do what it takes to regain and maintain the public trust."
The police department said it was able to recover the jewelry and is "making arrangements" to return it to its owner.
Hill was placed on administrative leave pending the outcome the investigation.