The appointment takes effect immediately. Slatkin had already been scheduled to conduct the orchestra this fall.
Slatkin, an American, is a familiar figure in London: he was chief conductor of the BBC Symphony from 2000 to 2004, and first conducted the RPO 31 years ago.
"Nothing pleases me more than to return to this orchestra in this renewed relationship," Slatkin said. "Our concerts over the last few months were among the most pleasurable musical experiences I have had, and I anticipate several more years of exciting music-making with my colleagues."
"Maestro Slatkin's recent series of concerts with the orchestra was hugely successful, and the members of the RPO were unanimous in their decision to offer him a major conducting post," said managing director Ian Maclay. "We look forward to developing our relationship with Leonard both in the U.K. and abroad as he brings a breadth of repertoire to the orchestra which will prove invaluable."
Slatkin announced earlier this year that he would leave the National Symphony in 2008. He begins a two-year term as principal guest conductor of the Los Angeles Philharmonic's Hollywood Bowl concerts this summer.