The Brimmon Oak in Newtown - around which a new bypass will be built after a petition attracted 5,000 signatures - finished runner up to an even older oak from Poland.
It was named the UK's Tree of the Year in December.
Ancient tree campaigner Rob McBride, who attended the award ceremony in Brussels, said it was "amazing news".
The Brimmon Oak received about 16,200 votes, some 1,400 fewer than Poland's Josef Oak, in the results announced on Tuesday. The Lime Tree at Lipka, Czech Republic, came third with about 14,800 votes.
A 500-year-old oak in Powys has finished second in the European Tree of the Year award for 2017.