The bloc’s diplomatic leader said the unannounced inauguration, as well as his contested re-election last month, lacked “democratic legitimacy”.
The opposition says the poll was rigged in favor of Mr Lukashenko.
Mass protests continued in the capital, Minsk, after the inauguration. Acts of violence by the police have been reported.
The country’s electoral commission said Lukashenko, who has been in power for 26 years, won more than 80% of the vote, triggering weeks of protests in the former Soviet republic.
Several hundred people attended Lukashenko’s swearing-in ceremony at the Independence Palace on Wednesday, with many streets in Minsk being cordoned off.
The guests were mostly loyal officials, and apparently no foreign dignitaries were invited. An opposition member described it as a “meeting of thieves”.
Besides the 27 members of the European Union, the United States also rejected the election results. But Lukashenko continues to be supported by neighboring Russia, his most important ally.
What does the EU say?
“The European Union does not recognize their falsified results. On this basis, the so-called “inauguration” of September 23, 2020 and the new mandate claimed by Alexander Lukashenko have no democratic legitimacy, “the EU’s Josep Borrell said in a statement.
“This ‘inauguration’ directly contradicts the will of a large part of the Belarusian population, expressed in numerous peaceful demonstrations unprecedented since the elections, and only serves to further aggravate the political crisis in Belarus. “
He added that the EU was reviewing its relations with Belarus “in light of the current situation”.
What is the context?
Lukashenko, 66, insists he won the August 9 elections fairly and described the protests against him as a West-backed plot. Earlier this month, he secured a $ 1.5 billion (£ 1.2 billion) loan from Russia.
Mr Lukashenko’s main political rival, Svetlana Tikhanovskaya, who fled to neighboring Lithuania amid mass arrests, claims to have obtained 60-70% in places where the votes were correctly counted.
Many opposition figures are now in voluntary exile in neighboring countries amid a wave of arrests. But despite the current crackdown, anti-government protests have continued.