"There is no reason to believe that Poland was the target," Tusk told reporters after visiting the impact site near the eastern village of Tarnawa-Kolonia.
He said two airborne objects had crossed into Polish airspace during Russia's overnight barrage against western Ukraine.
One was confirmed to have entered Poland and left a crater at the crash site, while the second generated only a brief radar signal near the border and caused no consequences.
Tusk said the crater, measuring about 10 metres across and up to 5 metres deep, suggested the object had exploded on impact. There were no casualties or damage because it landed in an uninhabited area, reporters were told.
"We are not making a final determination, but according to military experts, the overwhelming majority of the evidence, including the recovered fragments, points to a Russian Kh-101 missile," he said.
Tusk reiterated that the Polish military had been prepared to shoot down the missile had it posed a direct threat to populated areas.
He said civilians had been warned by air raid sirens, adding that Poland's emergency warning procedures would continue to be refined as the country adapts to security risks stemming from the war in neighbouring Ukraine.
Earlier on Thursday, the Operational Command of Poland's Armed Forces said an unidentified object was detected in Polish airspace at 3:40 a.m. local time.
An F-16 fighter jet was scrambled to intercept it, but the object disappeared from radar before it could be positively identified. A military helicopter later located the probable impact site near Tarnawa-Kolonia.
Tusk said the United States had expressed interest in taking part in the investigation into the incident, while Ukraine had also offered assistance.
Defence Minister Władysław Kosiniak-Kamysz said Washington wanted to help determine the nature of the object. He added that Poland was counting on support from its allies.
"NATO must demonstrate its strength, resilience and unity," Kosiniak-Kamysz said.
Tusk said he had received messages of solidarity and offers of assistance from European leaders and other allies.
"We certainly will not be alone in dealing with this problem," he said.
The prime minister also announced a comprehensive review of Poland's response to the incident, including the actions of civilian and military authorities, to improve procedures for handling any future airspace violations.
"This war is full of surprises, and we have to learn very quickly how to respond to new situations and potential new threats," Tusk said.
"Based on the reports I have received, particularly from the military, everything worked as it should have," he added.
Earlier on Thursday, Ukrainian Foreign Minister Andrii Sybiha wrote on X that "a Russian Kh-101 cruise missile crossed into Poland" overnight, violating NATO airspace, as part of Russia's massive strike against Ukraine.
"This is yet another clear demonstration that strengthening Ukraine's air defence now is urgent and serves as a guarantee for the protection of the entire Euro-Atlantic community," Sybiha added.
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Source: IAR, PAP, polskieradio24.pl
Click on the audio player above for a report by Piotr Urbaniak