In its quarterly briefing, the commodities broker boosted its 2017/18 (Oct/Sept) deficit forecast from a previously estimated 3 million bags.
Marex said the deficit, which could be the fourth consecutive one, is expected to reflect a squeeze in both robusta and arabica, although it did not provide a breakdown.
"It's in multiple year deficits and we think it's about to go into another big deficit next year," said James Hearn, co-head of the agricultural brokerage.
The group pegged global coffee supply in 2017/18 at 151.8 million bags, down from 154 million in 2016/17, while demand was seen at 156.1 million, up from 154.1 million.
Marex had previously forecast supply at a higher 152.6 million and demand slightly lower, at 155.6 million.
The revision is mainly driven by lower production in key growers including Brazil, where the output estimate was cut by another half a million bags.
Indonesian production was seen at 10.5 million bags, in line with 2016/17 output but lower than the 10.9 million initially expected.
Citing poor weather, the broker also revised down its forecast for the Ivory Coast to 1.4 million from 1.8 million. However, this is still a rise on the 1 million expected in 2016/2017.
Vietnam is expected to recover in 2017/18, with output seen unchanged at 28 million bags, from 24.5 million in 2016/17.
But Marex noted the global shortfall in 2017/18 will be more difficult to plug because Brazil already released its government stocks to ease this year's shortfall to just 0.1 million.
"With that Brazil government release, we had a balanced market," Hearn said. "Now, we think that the cupboard in Brazil is empty ... We don't think they have anything else."