Alex, riding for the Marc VDS Moto2 team with whom he is set to stay next year to defend the title, finished second to South African race winner Brad Binder on Sunday to be sure of the crown with a round to spare and an unbeatable lead of 28 points.
His title repeated the brothers' 2014 feat of both winning world championships in the same season.
"He is not 'the brother of'. He is Alex Marquez and he is the world champion two times in Moto3 and Moto2," said proud Marc, who celebrated with his sibling. (Reporting by Alan Baldwin in London, Editing by Alison Williams)