We report the synthesis of a novel heteroheptanuclear metal string complex (HMSC) [Ni₃Ru₂Ni₂(μ₇-teptra)₄(NCS)₂](PF₆) 1 supported by tetra-pyridyl-tri-amine (H₃teptra) ligands. We employed X-ray diffraction and other spectroscopic techniques to characterize the complex. The observed remarkably short Ru–Ru distance of 2.2499(3) Å for 1 is indicative of a unique metal–metal interaction in the mixed-valence [Ru₂]⁵⁺ (S = 3/2) unit. The complex exhibits a relatively high magnetic moment value of 4.55 B.M. at 4 K, which increases rapidly to 6.00 B.M. at 30 K and remains at 6.11 B.M. from 50 to 300 K as shown by SQUID measurements, indicating a high spin (S ≥ 3/2) system which is further supported by the analyses of EPR spectra at low temperatures. These magnetic behaviors can be ascribed to the result of spin-exchange interactions among multi-spin centers.