He was speaking after authorities found a crater and scattered debris in a field following reports of an explosion in the east of the country.
"So far, everything indicates that we are dealing with a Russian Kh-101 cruise missile, but we are not getting ahead of the results of the detailed investigation," Tusk told a meeting of a government coordination team in the eastern city of Lublin.
He said the Polish military had been prepared to shoot down the missile had it continued its flight over Polish territory.
"We were ready to intercept the missile if it had continued its flight," Tusk said.
The missile came down near the village of Tarnawa-Kolonia in Poland's eastern Lubelskie region. There were no casualties or damage because it landed in an uninhabited area, Tusk said.
He added that there had been no problems with coordination among state institutions and that he had remained in close contact with Defence Minister Władysław Kosiniak-Kamysz and President Karol Nawrocki throughout the incident.
Tusk also said Ukrainian fighter jets had attempted to intercept missiles heading toward the Polish border, calling the incident another reminder that supporting Ukraine's air defences directly enhances Poland's security.
"This shows how important cooperation with our allies is," he said, adding that helping Ukraine destroy Russian missiles and drones over its own territory has a direct impact on Poland's safety.
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.
Poland's Operational Command said an unidentified object had been detected in Polish airspace at 3:40 a.m. local time during a massive Russian missile attack on western Ukraine.
An F-16 fighter jet was scrambled to intercept it, but the object disappeared from radar before it could be identified. A military helicopter later located the probable impact site near Tarnawa-Kolonia.
Kosiniak-Kamysz said Poland had placed all of its air defence systems on the highest state of readiness overnight after receiving advance warning of the Russian attack.
Polish F-16s, surveillance aircraft, NATO aerial refueling aircraft and ground-based air defence systems were activated, while more than 20 airborne objects came close to Polish airspace, he said.
He said investigators were continuing pyrotechnic and forensic examinations at the crash site to identify the object, adding that the Kh-101 cruise missile was the most likely explanation but that the hypothesis still required confirmation.
He also said that NATO commanders had been kept informed and that Poland would provide the alliance with a full report.
Operational Commander Gen. Ireneusz Nowak said an F-16 had reached the area just as the object struck the ground.
"Had the missile continued its flight, I would have ordered it to be shot down," Nowak said.
Meanwhile, prosecutors said about 100 personnel from the military, police and other security services were working at the impact site, where a crater about 10 metres wide and 5 metres deep was found.
Debris from the object was scattered across an area of about 200 metres.
The site of the incident in the village of Tarnawa-Kolonia in Poland's eastern Lubelskie region. Photos: PAP/Wojtek Jargiło
Ukrainian authorities have formally offered to assist Polish investigators, citing their experience in examining similar incidents. Polish prosecutors said they were are considering the request.
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Source: IAR, PAP, polskieradio24.pl
Click on the audio player above for a report by Piotr Urbaniak