The Russian foreign ministry acted against Waclaw Radziwinowicz because Poland expelled a Russian journalist.
In a message on Facebook, the ministry addressed the cat, Marusia, saying: "Don't worry... we'll look after you".
But Radziwinowicz, who works for the Gazeta Wyborcza daily, appeared unamused by the joke, saying: "Marusia is going with me to Poland".
Speaking to the BBC by phone from Moscow, he said: "I don't understand the [ministry's] message." He complained of "a kind of paranoia" surrounding his expulsion.
Sounding upset, he declined to give any further comment.
The ministry's message said Radziwinowicz had asked the ministry "to look after my cat, who is staying here".
It said he had given various interviews about his imminent expulsion, but "for some reason" had not mentioned that request about his cat.
"A final request is sacred - that's why we got concerned [about his cat] right away," it said.
"Masya, don't worry, if after all he abandons you or, God forbid, if something worse happens, we'll look after you. We promised Waclaw that we wouldn't abandon his cat."
On Friday the ministry ordered Radziwinowicz to leave Russia within 30 days. It was a "reciprocal measure", it said, because of Poland's expulsion of Leonid Sviridov, a reporter with RIA Novosti, part of the Kremlin-backed Rossiya Segodnya media group.
Sviridov left Poland on 12 December, after the authorities described him as a "danger to the Polish state". The expulsion followed an investigation by Poland's Internal Security Agency.
In a statement to the BBC, Gazeta Wyborcza's managing editor Roman Imielski said he did not understand Russia's "game" over Radziwinowicz, who has been the paper's Moscow correspondent for 20 years.
Imielski said the foreign ministry had not given Radziwinowicz any written notification - "only the words from an official of the Russian MFA [foreign ministry]".
"Waclaw will take the cat with him. In my opinion, the Facebook statement about Marusia is a kind of Russian humour," he added.
He called the expulsion "pure revenge on the best-known Polish journalist in Russia".
"Sviridov worked for a state news agency, Radziwinowicz - for a private newspaper. Waclaw for years was particularly disliked by the Kremlin. Not only for his critical articles about Putin and his people," Imielski said.
Radziwinowicz had exposed "corruption, state capture by a narrow group of oligarchs, intrusive propaganda and an attempt to rebuild the Soviet empire", he said.
"Waclaw is a real institution - his Moscow flat was always full of opposition people, human rights activists and NGOs."
A new Russian foreign ministry statement says it would be "justified to blame the previous Polish government for the deterioration of relations [with Russia]". "But the representatives of the country's new government are criticising Russia even more sharply," it complained.
Russia is expelling a veteran Polish correspondent - and has joked that it will look after his cat for him.