A selenium-substituted phosphaalkene, (Me3Si)2C=PSeTrp (1, Trp: 9-triptycyl), was synthesized by two methods, substitution reaction of (Me3Si)2C=PCl (3) with TrpSeLi, and treatment of TsiP(Cl)SeTrp (4, Tsi: C(SiMe3)3) with a catalytic amount of AlCl3. The newly obtained phosphaalkene was characterized in detail.