De Rugy won 3.82 per cent of the vote in those January primaries and has been critical of Hamon's policies.
French leaders try to stem fury over alleged police rape
Most recent opinion polls put Macron among the leading candidates in a close-fought presidential race.
"For me, it's not unity for its own sake that I see, I've been saying I want the unity of the left and of progressive forces in which I include Emmanuel Macron," de Rugy said on France Info radio.
"That's why I draw this conclusion today that I want to take part in the renewal of politics that Emmanuel Macron has launched," he added.
Environmentalist politics are split in the country. Greens party candidate Yannick Jadot, who is standing for the presidency himself and whom polls show picking up between one and two per cent of the vote, is in talks with Hamon about an electoral pact under which Jadot would withdraw in Hamon's favour.
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