The 28-year-old joins the club on a four-year contract and follows Lukasz Fabianski, Issa Diop, Jack Wilshere and Ryan Fredericks to the London Stadium as part of the Manuel Pellegrini revolution.
Another man that may well be coming in for the Hammers is Lazio winger Felipe Anderson, with BBC Sport reporting that the club are ready to smash their transfer record with a £40 million deal.
Anderson, who can play as a central attacking midfielder or a winger, has made 177 appearances for the Italian side and scored 34 goals, and is also an Olympic gold medal winner with Brazil after featuring in Rio 2016.
If West Ham were to land both Yarmolenko and Anderson that would surely kill off the speculation that has been linking Dimitri Payet with a sensational return to the club.
As reported by the Mirror last week, the Irons are supposedly interested in a reunion with Payet, a player who once lit up the Boleyn Ground and the London Stadium, before leaving in acrimonious circumstances 18 months ago.
Opinion is divided about the idea of the player returning, but surely that prospect goes out the window the second Anderson walks through the door.
There would simply be too much competition to keep everybody happy. Payet would definitely play, but that would leave one of Yarmolenko and Anderson on the behch. Robert Snodgrass, Arthur Masuaku and Michail Antonio are also there for competition.
The Frenchman has been in brilliant form since returning to Marseille, but if the Anderson deal goes through then it is difficult to see why West Ham would plot a swoop for their former hero.