Ostrovsky, a writer and producer for the New York-based Vice News Network, was among six journalists detained Monday night, although he was the only one not released immediately.
On Thursday, Vice News confirmed Ostrovsky's release.
Vice News is delighted to confirm that our colleague and friend Simon Ostrovsky has been safely released and is in good health. We would like to thank everyone for their support during this difficult time. Out of respect for Simon and his family's privacy, we have no further statement at this time.
CBC foreign correspondent in Moscow, Jean-François Bélanger tweeted photos of Ostrovsky minutes after his release, including commentary on his treatment.
Prior to his detention, Ostrovsky had been reporting about masked gunmen seizing buildings in eastern Ukraine. A spokeswoman for the pro-Russian separatists confirmed the American journalist's detention on Wednesday and that he was being held at Ukrainian security service building that had earlier been seized.
The details of Ostrovsky's release are not clear.
On Thursday, he was spotted by CBC's Jean-François Bélanger walking along the side of a road "with his belongings in a plastic bag." The CBC news car then drove Ostrovsky to Donetsk.