Major Francis Beckford Ward described the Crimean rat as "an audacious brute".
And in one of 70 letters to his parents he told of a fellow soldier being "put to flight" by hundreds of the huge rodents.
The 700 pages of correspondence, which also shed light on what the fighting man thought of Florence Nightingale, are part of a private collection to be sold at Phillips in London on 14 March. It is expected to fetch up to pounds 3,000.
The letters were all written between December 1854 and June 1856.
In one letter, the newly-promoted major in the Royal Artillery wrote: "Our chief enemies at this moment are the rats and they annoy us exceedingly. The Crimean rat seems to be a most audacious brute and he has no respect for persons or things, he eats everything he can get at, and he fights and squeals, and runs over one, as one is lying in bed, without the smallest compunction.
"My men are quite afraid of them and my late Sergeant Major (John Sweeney), a great stout man of 6ft 3ins . . . brought me home a wonderful story one night of being attacked on his road home from headquarters by hundreds of these animals and being fairly put to flight.
"He really grew quite pale in telling the tale!"
The major said that from the appearance of the dead and wounded the Russians were all badly clothed and were, almost without exception, either old men or young boys.
Most were suffering from dysentery and their only food appeared to be "a small bag of black mouldy bread with a little oil to moisten it".
The major, who was senior British officer at the Battle of Tchernaya on 16 August 1855, also refers somewhat dismissively to the pioneering nurse Florence Nightingale.
He wrote: "Miss Nightingale is here again and I understand that she threatens to pay a visit to my hospital during the present week."
Felix Pryor, manuscripts specialist at Phillips, said: "She was actually seen by many out there as an interfering pain in the neck.
"People were quite happy to continue things as they were. She was obsessed with hygiene and cleanliness. But in those days nurses tended to be regarded as drunken slouches, the lowest of the low."Reuse content