US coffee chain Starbucks is denying Ethiopia earnings of £47m ($88m) a year, according to Oxfam.
Starbucks says it has programmes in place to help farmers
The UK charity says Starbucks asked the National Coffee Association (NCA) to block the country's bid to trademark three types of coffee bean in the US.
Oxfam says poor farmers would have benefited from the move but the NCA says there is no economic case to support the charity's claims.
Starbucks denies initiating opposition to the trademark application.
The Ethiopian government filed its applications to trademark its most famous coffee bean names - Sidamo, Harar and Yirgacheffe - in US courts last year.
Oxfam claims that Starbucks flagged up the application to the NCA - of which the firm is a leading member.
The NCA then filed its opposition at the US Patent and Trademark Office.
Oxfam's Phil Bloomer said the charity had worked with Starbucks in the past and appealed to the firm to "act responsibly".
"Their behaviour on this occasion is a huge backwards step and raises serious questions about the depth of their commitment to the welfare of their suppliers," he said.
Berhanu Kebede, Ethiopia's ambassador to the UK, said in a statement that the international community should support the country's efforts to ensure farmers got a fair reward for their hard work.
"This campaign has a pivotal role in redressing the unfairness of the international trading system," he said.
Starbucks' senior vice-president of coffee procurement, Dub Hay, denied approaching the NCA to oppose the Ethiopian move.
"We did not get the NCA involved - in fact it was the other way around, they contacted us."
Robert Nelson, head of the NCA, backed Mr Hay's claim, adding that his organisation opposed the Ethiopian move for economic reasons.
"For the US industry to exist, we must have an economically stable coffee industry in the producing world," he said.
"This particular scheme is going to hurt the Ethiopian coffee farmers economically."
He claimed that the Ethiopian government was being badly advised, and the move would price them out of the market.
The trademark move would also reduce demand for the country's coffee, he said.
Oxfam said the NCA and Starbucks should not dictate to Ethiopia how best to sell its products.