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In this article we prove that the Klein-Gordon equation in the de Sitterspacetime obeys the Huygens' principle only if the physical mass $m$ of thescalar field and the dimension $n\geq 2$ of the spatial variable are tied bythe equation $m^2=(n^2-1)/4 $. Moreover, we define the incomplete Huygens'principle, which is the Huygens' principle restricted to the vanishing secondinitial datum, and then reveal that the massless scalar field in the de Sitterspacetime obeys the incomplete Huygens' principle and does not obey theHuygens' principle, for the dimensions $n=1,3$, only. Thus, in the de Sitterspacetime the existence of two different scalar fields (in fact, with m=0 and$m^2=(n^2-1)/4 $), which obey incomplete Huygens' principle, is equivalent tothe condition $n=3$ (in fact, the spatial dimension of the physical world). For$n=3$ these two values of the mass are the endpoints of the so-called inquantum field theory the Higuchi bound. The value $m^2=(n^2-1)/4 $ of thephysical mass allows us also to obtain complete asymptotic expansion of thesolution for the large time. Keywords: Huygens' Principle; Klein-GordonEquation; de Sitter spacetime; Higuchi Bound

Huygens' Principle for the Klein-Gordon equation in the de Sitter spacetime