An unidentified object that entered Polish airspace from the direction of Belarus was probably an observation balloon, Poland's defence ministry said on Saturday on Twitter.
NATO-member Poland is on alert for infractions of its airspace as war rages in neighbouring Ukraine after incidents in November and December in which missiles landed on its territory.
Captain Ewa Zlotnicka, press officer of the Armed Forces Operational Command, told broadcaster TVN 24 the object entered Polish airspace on Friday evening.
The ministry said radar contact with it was lost near Rypin, a town in north-central Poland. A Territorial Defence Force spokesman said a search for it was under way.
Russian and Belarusian authorities were not immediately available for comment.
After the deaths in November of two Poles hit by what Warsaw concluded was a misfired Ukrainian air defence missile, and the discovery of the remains of the missile in northern Poland in April, national security is a key issue in Poland ahead of elections later this year.