The track will be released on 12'' vinyl on November 14, backed by a remix by Doldrums, with 200 signed copies available at the New York independent label market tomorrow (October 8). All proceeds again go to Amnesty.
The inaugural Independent Label Market took place in London in May, with a number of indie labels setting up stalls in London's Berwick Street to sell records and other goods directly to the public.
XL sold a "start your own label" kit for sale that included the master tapes, lifetime copyright and one-page contract for an EP called Fresh Touch, which was recorded by label founder Richard Russell and XL's in-house engineer Rodaidh McDonald in Ethiopia.
The New York event will include stalls from XL, Domino, 4AD and DFA, as well as album face painting and a kid's disco.
Portishead, who are currently on tour in the US, played Chase The Tear on the Jimmy Fallon Show in the US on Wednesday night.
The song, which has a krautrock-esque edge, is said to give an idea of what to expect from the band's much-awaited fourth album, the follow up to 2008's Third.
Chase The Tear has sold almost 6,000 copies in the UK to date, while Third has shifted 137,000.