Two new isostructural layered oxohalides FeTe3O7X (X = Cl, Br) were synthesized by chemical vapor transport reactions, and their crystal structures and magnetic properties were characterized by single-crystal X-ray diffraction, Raman spectroscopy, magnetic susceptibility and magnetization measurements, and also by density functional theory (DFT) calculations of the electronic structure and the spin exchange parameters. FeTe3O7X crystallizes in the monoclinic space group P2(1)/c with the unit cell parameters a = 10.7938(5), b = 7.3586(4), c = 10.8714(6) angstrom, beta = 111.041(5)degrees, Z = 4 for FeTe3O7CI, and a = 11.0339(10), b = 7.3643(10), c = 10.8892(10) angstrom, beta = 109.598(10)degrees, Z = 4 for FeTe3O7Br. Each compound has one unique Fe3+ ion coordinating a distorted [FeO5] trigonal bipyramid. Two such groups share edges to form [Fe2O8] dimers that are isolated from each other by Te4+ ions. The high-temperature magnetic properties of the compounds as well as spectroscopic investigations are consistent with an isolated antiferromagnetic spin dimer model with almost similar spin gaps of similar to 35 K for X = Cl and Br, respectively. However, deviations at low temperatures in the magnetic susceptibility and the magnetization data indicate that the dimers couple via an interdimer coupling. This interpretation is also supported by DFT calculations which indicate an interdimer exchange which amounts to 25% and 10% of the intradimer exchange for X = Cl and Br, respectively. The magnetic properties support the counterion character and a weak integration of halide ions into the covalent network similar to that in many other oxohalides.