The Mint said the coin, which is only the second UK £20 coin it has ever made, has been struck to remember the bravery of those who answered the call to fight for king and country a century ago.
Created by sculptor John Bergdahl in the art deco style prevalent at the time, the coin depicts the figure of Britannia watching over the first troops leaving for France when the war started in 1914. It bears the current portrait of the Queen on its obverse by Ian Rank-Broadley.
Minted in fine silver, the new outbreak of war coin is limited to an issue of 250,000. It 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. That coin, which featured a St George and the Dragon design, was also limited to a run of 250,000 and has sold out.
Mr Bergdahl said: "I decided to use the figure of Britannia in order to represent all of the allies who took part, most of whom were British or drawn from the British Empire.
"I combined inspiration from two sources: the South African medal for war services and the First World War memorial plaque awarded to those who died, using an art deco style which I felt best reflected the period."
Shane Bissett, the Royal Mint's director of commemorative coin and bullion and medals, said: "The centenary of the outbreak of the First World War is an occasion of such significance, it was important to us that this coin should takes its place in the Royal Mint's programme of commemoration, remembering the bravery and sacrifice of those who answered the call to fight for king and country."