Adria announced that Soler died on Monday at his home in Rubi, near Barcelona.
Adria said on his official Twitter account that it was "the saddest news I wish that I have never had to bear."
On arriving at El Bulli in 1981, Soler was an essential figure in what became one of the world's most influential restaurants.
Alongside Adria, he revolutionized diners' experiences to the point that El Bulli maintained the almost unattainable Michelin three-star status for over a decade.
El Bulli, located in Montjoi cove in northeastern Spain, was closed in 2011 so as to start the Bulli Foundation, a food research centre.