Unionists claim that Britain leaving the EU - known as a Brexit - will cost the economy billions of pounds, around a million jobs, and lead to a decline in Britain's global influence.
But the agency said in a report, cited by the BBC, that the impact from a Brexit won't actually be that bad.
"Our central view is that the negative economic impact of Brexit would be relatively small," said Moody's.
"[We do] not expect to see significant increases in unemployment or [interest] rates, or substantial declines in property prices across the
"Although there are clear downside risks to the City of London's standing as a global financial centre, in our central scenario we do not see Brexit materially damaging its strong position."
Britain will vote on June 23 this year on whether the UK should leave or stay in the European Union.
The comments come only a day after the Confederation of British Industry (CBI) - the lobby group which speaks for 250,000 public and private sector companies - laid out its doomsday scenario for Britain.
The director-general of the Confederation of British Industry (CBI) , Carolyn Fairbairn, said at the weekend that "leaving the EU would cause a serious shock to the UK economy, with a potential cost to UK GDP of £100 billion and 950,000 jobs by 2020 and negative echoes that could last many years after that." She made her comments in a lecture at the London Business School, citing a PwC analysis.
On Sunday, former Prime Minister (1990 to 1997) John Major said in an opinion piece for the right-leaning newspaper The Telegraph that Britain needs the 28-nation bloc. If it wasn't for the EU, the UK would still be considered the "sick man of Europe," he said. He also unequivocally stated the "leave" campaign was deluded in how it presented a UK outside the bloc
Both Fairbairn and Major cited damage to trade ties as an issue for Britain following a Brexit.
However, Matthew Elliott, chief executive of Vote Leave, spoke to the BBC after viewing the Moody's report and said "the UK will not face trade barriers after we Vote Leave, jobs will be safe and our credit rating will not be affected."