The Atletico Madrid striker has struggled with injury after suffering a hamstring injury in early May - an ailment that limited him to just nine minutes in the Champions League final.
However, Del Bosque says Costa is now fit again and will state his case for a first-team spot for the tournament, where they will face Australia, Chile and the Netherlands in Group B.
"Diego will play against El Salvador," the former Real Madrid told reporters.
"He had a good training session and felt good. We want him to play and check how he feels, especially after a while since the last time he played a complete match.
"If we thought that there was a risk for Diego Costa playing then we would not use him. We think that he's fine, his medical situation is done, he's ready so we want him to play."
Del Bosque also made clear that he would make significant changes to his starting XI after the 2-0 win over Bolivia last time out.
"We are in our second week of training and we are going to use specially those players who weren't able to be here in the first week."
However, the situation of striker David Villa, who has signed for MLS outfit New York City FC and also secured a guest stint at A-League side Melbourne City, was of no concern to the 63-year-old.
He continued: "We are specifically focused on the World Cup, not what is in their future.
"They have their own agents who look after their interests and it's been a fact that several players that were in our league are leaving to other leagues - David's situation is not unique."