ST. PETERSBURG, Russia—French President Emmanuel Macron drew a spotlight to growing discord between U.S. and European foreign policy Friday, touting a new multibillion investment in Russia by his country’s top oil company and joining President Vladimir Putin in defense of the Iran nuclear deal.

Appearing alongside Mr. Putin at Russia’s main economic forum Friday, Mr. Macron said French major Total SA’s estimated $2.6 billion investment in a giant Arctic natural-gas facility would help French energy security and pave the way...