Debbie Wasserman Schultz's move follows a leak of emails appearing to suggest that party insiders tried to thwart the campaign by Hillary Clinton's rival.
Bernie Sanders had pressed for the party chairwoman to step down.
Mrs Clinton is to be officially nominated at the Philadelphia meeting.
The Vermont senator had said Ms Wasserman Schultz "should not be chair" of the Democratic National Committee.
"And I think these emails reiterate that reason why she should not be chair," he told ABC's This Week programme.