President Donald Trump is expected to seek Moscow’s help in scaling back Iran’s military presence in Syria when he meets in Helsinki with Russian President Vladimir Putin on July 16 for a summit, say European diplomats and U.S. officials.

Some American officials see a potential for a U.S. and Russian “convergence” on the issue that Mr. Trump could explore at the summit meeting, said one senior diplomat, one of several who have recently spoken to American officials on the issue.

Secretary...