Kay Swinburne told a BBC Radio Wales debate that "not a single kilogram" of Welsh lamb is sold to the US.
Meanwhile UKIP AM Mark Reckless said he would like to see direct support to farmers, with less bureaucracy.
He said the EU had still not negotiated a US trade deal despite 40 years of UK membership.
"Welsh Lamb is a huge export for us. It really is critically important for our rural communities," Ms Swinburne said.
She said the biggest market was the EU, but warned of high EU tariffs if the UK left the bloc. "If we come out, no matter what the tariff on our Welsh lamb will be above 60%," she said.
"That means we will not be able to sell our lamb into the one market that we have right now.
"We have not sold a single kilogram of lamb to the United States of late.
"So, we have no market for it. That would be devastating for our communities.
"I don't believe any government in Westminster will look after our farming community here in Wales in the way the EU does, because they are farming led."
Mr Reckless said he would would like to see a system "where we have freer trade generally" and deals are "negotiate those trade deals with other countries".
"I'd like to give direct support to farmers, at least as much as they get now from the [Common Agricultural Policy], with much less bureaucracy associated with it," he said.
"If we are willing to open up our agricultural market on a reciprocal basis while giving farmers direct support, then that will make it much easier for us to do a much broader and deeper free trade deals than we have been able to with the European Union."
He said the EU still, after 40 years of our membership "hasn't negotiated a deal with the US".
Welsh lamb would be left without a market if Britain left the European Union, a Remain-supporting Welsh Tory MEP has said.