His release throws into disarray a bid by Lithuanian authorities to probe the banker over 14.5 million euro embezzlement allegations.
Romanov, 66, had been held in detention under guard by armed police in a Moscow hospital since Monday after his arrest on a Lithuanian extradition warrant.
Last night, he walked as a free man from Izmailovsky district court, into a waiting ambulance.
Friends said he was suffering from ill health linked to a heart complaint that resurfaced when he was suddenly held by police in the Russian capital.
As he sat in an iron "cage" in the court room, Romanov's lawyers argued that the probe into financial irregularities linked to his former Ukio Bankas role was based on a "political vendetta" against him.
"In Lithuania, Romanov is pursued for political motives. We submitted some documents proving it," said his lawyer Vladimir Filimonov.
The hearing was closed, but it is understood judge Elena Pospelova considered the case for three hours before ruling there were significant "irregularities" with the extradition paperwork provided by the Lithuanian authorities.