Alvaro Saborio misses out after damaging his foot during training, with Carlos Hernandez and Kendall Watson also omitted from the final 23-man group
Ruiz, on loan at PSV from Fulham, and Campbell are likely to play key roles in Jorge Luis Pinto's team in a Group D featuring Uruguay, Italy and England.
Arsenal-owned forward Campbell, who scored a famous goal against Manchester United while on loan at Olympiakos this season, is a 31-time international with nine goals to his name.
Everton's Bryan Oviedo missed the initial cut after failing to recover from a broken leg, while leading active goalscorer Alvaro Saborio is also out after damaging his foot during the training camp.
Midfielder Carlos Hernandez and Deportivo Saprissa defender Kendall Watson are the other two players to miss out on the final party heading to Brazil.
Costa Rica have a friendly against the Republic of Ireland in the United States before their World Cup begins with a clash against Uruguay on June 14.
Squad in full:
Goalkeepers: Daniel Cambronero (Herediano), Keylor Navas (Levante), Patrick Pemberton (Alajuelense)
Defenders: Johnny Acosta (Alajuelense), Junior Diaz (Mainz), Oscar Duarte (Club Brugge), Waylon Francis (Columbus Crew), Christian Gamboa (Rosenborg), Giancarlo Gonzalez (Columbus Crew), Roy Miller (New York Red Bulls), Heiner Mora (Deportivo Saprissa), Michael Umana (Deportivo Saprissa)
Midfielders: Michael Barrantes (Aalesund), Christian Bolanos (Copenhagen), Diego Calvo (Valerenga), Jose Miguel Cubero (Herediano), Esteban Granados (Herediano), Yeltsin Tejeda (Deportivo Saprissa)
Forwards: Celso Borges (AIK), Randall Brenes (Cartagines), Joel Campbell (Arsenal), Bryan Ruiz (Fulham), Marco Urena (Kuban Krasnodar).