Acetylsalicylic acid reacts with gadolinium(III) ions to yield a new gadolinium complex (1) displaying a chain structure which has been determined by single crystal X-ray crystallography: monoclinic P2₁/n (no. 14), a = 13.5709(13), b = 6.7141(9), c = 22.7232(17) Å, β = 99.705(14)°, Z = 4. The thermal dependence of the χ_MT product (χ_M = molar magnetic susceptibility) has been measured and analysed according to the analytical expression of an infinite chain of classical spins derived by Fisher. The Gd⋯Gd interaction is weak but surprisingly is ferromagnetic with a magnitude of 0.037 cm⁻¹. Modelling of the field dependence of the magnetization at 2 K corroborates this result. It should be emphasized that (1) is the first example of a polynuclear gadolinium complex displaying a ferromagnetic Gd⋯Gd interaction and having a fully determined structure.