He and two other MEPs were secretly filmed in 2010 by undercover reporters from Britain's Sunday Times newspaper discussing payments in return for tweaking EU laws.
Severin, a former foreign minister, has always maintained his innocence but a Romanian court convicted the 62-year-old in February and sentenced him to three years.
"What began as a journalistic farce ends with a judicial farce," Severin said on Facebook after the high court in Bucharest rejected his appeal.
Severin agreed to put forward amendments to EU legislation on behalf of the reporters, who were posing as lobbyists and who secretly filmed their conversations, in exchange for money.
The two other MEPs caught in the sting, former Austrian interior minister Ernst Strasser and former Slovenian foreign minister Zoran Thaler have already been convicted.