Due to the delicate state of the notebooks and journals, the 4,100-page archive was until now only accessible to the poet's official biographer.
But Cambridge University Library, which bought the collection five years ago, has now digitalised the treasure trove and made it public online at
In scrawled ink now almost a century old in places, the booklets are filled with names and addresses, scribbled drawings and notes, draft poetry and diary entries of Sassoon's life as he went to war.
He describes his first day at the Somme as a "sunlit picture of hell".
"Since 6:30 there has been hell let loose," reads one damp-stained entry from July 1 1916.
"The air vibrates with the incessant din - the whole earth shakes and rocks and throbs. It is one continuous roar - machine guns tap and rattle - bullets whistling over head."
The archives also contain drafts of his letter 'A Soldier's Declaration', in which he said the war was "being deliberately prolonged by those who have the power to end it" and which brought him to national prominence when it was read out in parliament and printed in the press.
"The war diaries Sassoon kept on the Western Front and in Palestine are of the greatest significance, both nationally and internationally, and we are honoured to be able to make them available to everyone, anywhere in the world, on the 100th anniversary of the First World War," said Cambridge University librarian Anne Jarvis.
"From his Soldier's Declaration to his eyewitness accounts of the first day of battle on the Somme, the Sassoon archive is a collection of towering importance, not just to historians, but to anyone seeking to understand the horror, bravery and futility of the First World War as experienced by those on the front lines and in the trenches."
One poetry notebook from August 1918 contains an early draft of his celebrated poem 'The Dug-Out' with an additional verse.
Small enough to be carried in Sassoon's army uniform, the books contain drawings of his time in France and record the moment he was shot by a sniper at the Battle of Arras.
They cover the period 1915-1927 and 1931-1932. Considered one of the foremost of the English war poets and decorated for his bravery, Sassoon died in 1967 aged 80.