The half-diminished seventh chord belongs to the most commonly utilized elements of Sibelius's harmonic vocabulary. It can be found anywhere already in Sibelius's early works. The chord is used both as a stylistic marker and a tool for analysis, while it gives a better comprehension of the composer's handling of tonality. The purpose of this paper is to show how the half-diminished seventh chord is used within Sibelius's harmony, as well as its interaction between modality and tonality. The modal color evidences how Sibelius is indebted to the Romantic composers. However, once the distance between Sibelius and his major influences (Wagner, Grieg, Mussorgsky) is reevaluated, the half-diminished seventh chord sonority turns out to be a powerful element of his style.