The court confirmed the sentence against Carlos Oswaldo Alvarado of the Barrio 18 gang on Thursday. It was handed down Wednesday.
The attack occurred in 2010 in the suburb of Mejicanos in retaliation for the killing of Alvarado's brother by members of the rival Mara Salvatrucha gang.
Gang members stopped the bus, soaked it in gasoline and ignited it.
The sentence is largely symbolic because the maximum prison sentence anyone can serve is 60 years. El Salvador's street gangs claim some 70,000 members and are largely responsible for the country's soaring murder rate.