The Mint said the coin, which is only the second £20 coin it has ever made, has been struck to remember those who fought in the conflict.
Created by sculptor John Bergdahl in an art deco style, the coin depicts the figure of Britannia watching over troops leaving for France at the war's outbreak in 1914.
It bears the current portrait of the Queen on its obverse by Ian Rank-Broadley. Minted in fine silver, the war coin is limited to an issue of 250,000 and can be bought from the Royal Mint's website for its face value of £20.
The first UK £20 coin was released last year to celebrate the birth of Prince George its limited run of 250,000 sold out.
Mr Bergdahl said: "I combined inspiration from two sources: the South African medal for war services and the First World War memorial plaque."