A biohybrid catalyst where a metal complex is embedded in a cavity of a protein can be regarded as an artificial metalloenzyme. In particular, incorporation of synthetic organometallic species in a protein matrix will promote catalytic reactions which cannot be seen in nature. This chapter focuses on olefin metathesis via C=C bond formation/cleavage catalyzed by biohybrid catalysts consisting of Hoveyda-Grubbs (HG) type Ru complex attached to protein. Robust β-barrel proteins, nitrobindin and FhuA, and α-chymotrypsin with a deep cleft are considered as the protein scaffold in this chapter. The HG-type complexes introduced into the scaffolds are found to catalyze ring-opening metathesis polymerization and ring-closing metathesis with good chemoselectivity. This strategy will serve as a new method to construct an attractive biohybrid catalyst for abiological transformations in aqueous media.