Mr Medvedev has been Russia's prime minister since 2012
The Twitter account of Russia's prime minister was hacked on Thursday. The infiltrators posted a message saying Dmitry Medvedev had resigned.
The Russian-language feed, which has more than 2.5 million followers, was also filled with tweets denouncing the country's president, Vladimir Putin.
The impersonator wrote that Mr Medvedev would be pursuing a new career as a freelance photographer.
A Russian hacking collective has claimed responsibility for the attack.
The government quickly confirmed the account had been hacked.
"I resign. I am ashamed of the government's actions. I'm sorry," the infiltrators initially wrote, following it up with tweets saying that electricity would be banned, and that Vladimir Putin was "wrong".
Shaltay-Boltay, a Russian hacking collective, has said it carried out the attack.
The group, whose name is Russian for Humpty Dumpty, also claimed it had infiltrated the Gmail account and iPhones of the prime minister.
Following the incident, it tweeted: "Several mail accounts - including the Gmail - and the content of three iPhones of a certain prime minister have randomly fallen into our hands on the internet. Details later."
Mr Medvedev's English-language Twitter account did not appear to be affected by the attack.