Ronaldo has 226 caps and 143 goals for Portugal, both all-time records for international football. He scored 25 goals in his last 30 international appearances ahead of the tournament, a return that Martínez cited as evidence the forward remains as important as ever.
Speaking to Cadena SER, the Portuguese head coach was unequivocal about Ronaldo's standing in the squad.
"He is an icon. What he has achieved in football is very significant and will continue to be so for many years to come. He is our captain, and he possesses the same competitive spirit and the same high standards as the rest of the players.
"He has scored 25 goals in his last 30 games. He is very important for us and has a decisive influence. The long career he has had should be taken advantage of, and we hope it can be a unique experience."
Portugal open their Group K campaign against DR Congo on June 17, before facing Uzbekistan and Colombia. The squad will also play warm-up fixtures against Chile and Nigeria in early June ahead of travelling to the United States.
Martínez has previously stated that Ronaldo does not need to win the World Cup to be considered the greatest player of all time — the one major prize that has eluded him throughout a career that includes five Champions League titles, five Ballon d'Or awards and three league titles across England, Spain and Italy. He won the European Championship in 2016 and the Nations League twice.
The tournament in North America will be Al-Nassr star Ronaldo's last realistic chance at the trophy, though Martínez has declined to rule out a 2030 appearance, telling reporters that "no one should doubt" his ability to reach that point physically if the desire remains.