Reported to have Russian inscriptions on it, the object was discovered by foresters near the tiny village of Orzechowo on Wednesday night.
After being examined for possible signs of radiation, the area was secured by police and the object was taken away for examination.
Police spokesman Jacek Wilczewski said: “We have informed the ABW (Internal Security Agency) and SKW (Military Counterintelligence Service) and we are waiting for their decision.”
“We have taken the object (for testing), an investigation will be conducted under the supervision of the prosecutor's office.”
The discovery follows a similar incident in March when a farmer in the same region found an object in one of his fields.
It was later discovered to have been a weather balloon.
Another object found in a Polish forest in 2023 sparked alarm after it turned out to be the remains of a Russian KH-55 missile, which had most likely gone off course after being launched from Belarus.
At the end of last year, another Russian missile briefly entered Polish airspace before crossing the border into Ukraine, and in 2022, two Polish farmers were killed when a missile landed in their village close to the Ukrainian border.
Investigations revealed that the missile had been fired by Ukrainian air defense forces to repel a Russian missile attack.
Following the most recent discovery, police said: “What we are dealing with will probably become clear soon.”