The Georgian, a star for his country at Euro 2024, becomes Arne Slot's first signing at Anfield.
Mamardashvili is now officially a Liverpool player, but the 23-year-old has been immediately loaned back to Valencia.
With Brazil's Alisson still very much the Reds' number one, Mamardashvili will continue his development in Spain.
In 2021, the Georgian joined Valencia from Dinamo Tbilisi - where he had spent his formative years.
Mamardashvili graduated from the Tbilisi academy, which has also produced Khvicha Kvaratskhelia, in 2018.
He struggled to break into the first team of his boyhood club and was loaned out to Rustavi at the age of 19.
Mamardashvili then spent two years on loan at Locomotive Tbilisi between 2019 and 2021, before joining Valencia.
