Vibronic structures of the ÖX̃ and B̃–X̃ bands of the HgAr2 triatomic cluster were investigated by laser-induced fluorescence (LIF) spectroscopy. The LIF spectra of the ÖX̃ and B̃–X̃ bands of HgAr2 were observed free from hindrance of strong A–X and B–X transitions of HgAr by monitoring the fluorescence in the visible region. The analysis of the vibronic structures led to the fundamental frequencies of ω1=26.3(3), ω2=12.0(5), and ω3=28.1(4) cm−1 for the à state and ω1=9.2(3) and ω2=23.8(6) cm−1 for the B̃ state, where subscripts ‘‘1,’’ ‘‘2,’’ and ‘‘3,’’ respectively, denote symmetric stretching, bend, and asymmetric stretching modes. A number in parentheses represents an estimated limit of error (σ). It was found that HgAr2 in the B̃ state predissociates to produce HgAr(A)+Ar. The vibrational and rotational distributions of the HgAr fragment in the A state were derived by observing LIF spectra of the E–A band. Based on these distributions, level energies of HgAr2 in the B̃ state were estimated with respect to those of HgAr in the A state, and then, the dissociation energies (De) of HgAr2 to Hg+Ar+Ar were derived to be 374(2), 540(2), and 249(2) cm−1 for the X̃, Ã, and B̃ states, respectively. The vibronic structure of HgAr2 in the lowest Rydberg (Ẽ) state associated with Hg(7 3S1) was investigated by the optical–optical double resonance spectroscopy via the à state. The geometrical structure, in which an outer Ar atom is loosely bound to the Rydberg-type HgAr core, that is close to HgAr+, is proposed for the Ẽ state, and a presence of large amplitude motion of the outer Ar atom is deduced from the obtained fundamental frequencies of ω1=102.1(3) and ω2=5.4(2) cm−1, where subscripts ‘‘1’’ and ‘‘2’’ denote a stretching mode of the HgAr core and the Ar–Hg-Ar bending mode.