The 1-arsa-3-germaallene Tip(t-Bu)Ge=C=AsMes* (1, Tip = 2,4,6-triisopropylphenyl, Mes* = 2,4,6-tri-tert-butylphenyl), a stable heavier group 14 and 15 congener of allenes, has been synthesized by debromofluorination of Tip(t-Bu)Ge(F)-C(Br)=AsMes*. It reacts with methanol and 2,3-dimethyl-1,3-butadiene by the Ge=C double bond. The allenic-type structure of 1, featuring cumulated Ge=C and C=As double bonds, has been evidenced by means of spectroscopic and single-crystal X-ray determination. The electronic properties involved in this new system were obtained from DFT calculations. The mechanism of the reaction between 1 and the dimethylbutadiene is also described to understand the observed regio- and chemoselectivity.