The Girona-based restaurant, known for its dishes inspired by perfumes, took the top spot from Danish foraging restaurant Noma in 2013.
The World's 50 Best Restaurants list has been compiled by more than 900 international experts for Restaurant magazine.
The results were to be announced in a ceremony attended by top chefs from around the world at London's Guildhall at 2030 GMT.
At El Celler, the eldest brother Joan Roca heads the kitchen, while the youngest Jordi is pastry chef and Josep is head sommelier and in charge of front of house.
"It's been a magical year, full of projects, emotion, sharing and dreams becoming reality," Joan Roca told a press conference in London on Monday ahead of the announcement of the new results.
The magazine praised the restaurant last year for making "emotional cuisine" which takes diners back to childhood memories.
Since the awards were established in 2002 only five restaurants have held the title: Spain's El Bulli, in 2002 and from 2006-2009; California's French Laundry in 2003 and 2004; Britain's Fat Duck in 2005; Noma in Copenhagen from 2010 to 2012, and El Celler de Can Roca last year.
Spain dominated last year's top 10 with Mugaritz in San Sebastian at number four, and Arzak, also in San Sebastian, at number eight.
Italy's Osteria Francescana was in third place, New York's Eleven Madison Park in fifth, D.O.M. in Sao Paulo sixth, Dinner by Heston Blumenthal in London seventh, Steirereck in Vienna in ninth and Vendome in Bergisch Gladbach, Germany, in 10th.
South America had a record six restaurants in last year's top 50, while Asia boasted seven and the United States and France had six each.