The youngster arrived at the Buenos Aires club from a Cannes football academy, and will begin training with his new club at the beginning of next year.
River have been tracking the 14-year-old for some time and according to the father of Nicolas Sumsky, Victor, he will be worth "$30 million" in a couple of years.
Despite having attracted interest from several European clubs the player chose to sign for River Plate, because his father believes more players triumph at a younger age in South America.
"The teams who want him from there [Europe] normally spend millions of dollars on stars to reinforce their teams, and for my son it would be more difficult to triumph at 17 or 18 years of age," explained Victor.
"I think after going to River, another world power, in three or four years he will be prepared to return to Europe," he added.
Nicolas tried out for River's arch-rivals Boca Juniors five months ago, but although Boca were keen to sign him they didn't come to an agreement.
Several incredibly talented youngsters have emerged from River's youth ranks, the latest of these being current first-team attacker Diego Buonanotte.
In recent years, River's youth categories have signed players from such places like Uruguay, Paraguay, Colombia, Austria, Japan, and the United States.