Harald Espenhahn was convicted by a court in Turin in the north of the country of "voluntary homicide", a first in Italy for a workplace accident.
Seven workers died days and weeks after suffering severe burns when a fire broke out at the steel plant's thermal treatment department.
The fire was one of the worst in Italy and sparked a public debate over health and safety regulations.
Four other executives were convicted of complicity by carelessness and sentenced to 13.5 years in prison, and a fifth received a 10-year jail term. ThyssenKrupp was also fined one million euros.
The steelmaker called the verdict against Espenhahn "incomprehensible and in explicable," in a statement released in Italy, adding that its attorneys would look into what can be done about it.
The defence had claimed the trial was "political."