The conservative Republicans and their allies would form the largest opposition bloc, with 125 to 131 seats.
It is a stunning result for Mr Macron, after a presidential election that saw the collapse of both the two traditional main parties.
Marine Le Pen celebrated a political first after she was finally elected to the French Assembly in yesterday's second-round parliamentary election, though her personal victory stood at odds with the poor performance of her right-wing National Front party.
The result did show that Le Pen had made some headway in bringing her anti-EU, anti-immigration party closer to power, as its seat count was up six from the last election in 2012.
Paris - French voters on returned to the polls on Sunday for the second round of a parliamentary election, which President Emmanuel Macron's youthful party is tipped to win by a landslide, completing his reset of national politics.
The Socialist party was the biggest loser, expecting to shed more than 200 seats and hold only around 34 seats - again, better than forecast, but still a drubbing.
He says the party will have to rebuild from the top down, suggesting what he terms "the left" needs to change radically if it still wants to be a strong force in French politics.
Macron's clear majority will make it much easier to make changes to labour laws, employment policies etc, since he will not need to go through very long Parliament negotiations. "The president of the Republic has all the powers", Jean-Christophe Cambadelis said after announcing he would step down as Socialist Party chief.
In winning the presidency in May, he filled a political vacuum created by disarray within the Socialist Party and the Republicans, with Sunday night capping a sequence of events that looked improbable a year ago. It's a largely symbolic move required after legislative elections.
With the 42 seats of its allies MoDem, REM has a total of 350, giving it a powerful platform to launch Macron's slate of pre-business reforms.
Valls, a centrist member of the Socialist Party who previous year in a speech called anti-Zionism a form of anti-Semitism and who in 2009 said he has an "eternal bond" with the Jewish people because of his marriage to a Jewish woman, narrowly beat the far-left politician Jean-Luc Melenchon, who has been accused of espousing anti-Semitic rhetoric in speeches, in the Essone region in northern France.
Melenchon's resurgent France Unbowed and the Communist Party were on course to win 26 to 30 seats.
They include 27-year-old Rwandan orphan Herve Berville, who cruised to victory in the western region of Brittany, and woman bullfighter Marie Sara, who came within 100 votes of unseating senior FN figure Gilbert Collard in southern France.
The Socialist Party, which dominated the outgoing Assembly, was flattened by the unpopularity of former President Francois Hollande.