There has been no public comment from Julen Lopetegui, who was sacked this morning.
Hierro will be in the dugout on Friday when Spain begin their campaign in the Group B opener against Portugal and will remain in charge throughout the World Cup in Russia.
The former Spain and Real Madrid captain will be presented to the press at the Krasnodar Stadium later on Wednesday before settling in to his new role.
Spain and Real Madrid captain Sergio Ramos has issued a rallying cry on his twitter account to his Spanish teammates saying: “We represent the badge, the colours, the fans, the country.
“Responsibility and commitment are with you. Yesterday, today and tomorrow.”