The Spain international, who has just arrived in Brazil with the world and European champions for the Confederations Cup, has rejected the chance to renew his deal with the La Liga champions beyond 2014.
The 31-year-old has said he would like to experience different leagues and cultures after almost a decade as Barca's number one choice, which raised the possibility of him leaving during the closed season.
"Victor Valdes's representative has officially told [sports director Andoni] Zubizarreta that the player will complete his contract," Rosell told a news conference, as he reviewed the club's season.
"We are delighted he will be with us next year."
"I believe we will sign one or two more players," he said. "That also depends on whether players leave."
The prosecutor's office for tax crimes in Catalonia has accused them of filing fraudulent tax returns for the years 2006 to 2009 and of owing more than four million euros.
Messi, who is due to play for Argentina in a friendly in Guatemala on Friday, has denied wrongdoing, and Rosell backed the club's leading scorer.
"Messi's family is quite calm about it," he said. "They have our support. I have no doubt about his innocence."
