The geometrically frustrated diamond spin chain system has yielded materials with a diversity of interesting magnetic properties but is predominantly limited to compounds with single-spin components. Here, we report the compound [(CH3)2NH2]6[FeIII4FeII2(μ3-O)2(μ3-OH)2(μ3-SO4)8] (1), which features the mixed-valent iron(II/III) diamond chain: ∞[FeIII-(FeIII)2-FeIII-(FeII)2]. 57Fe Mössbauer spectroscopy shows that two-thirds of the total spins in the ∞[FeIII4FeII2] diamond chain are spin-5/2 (high-spin FeIII), while the remaining one-third are spin-2 (high-spin FeII). To date, 1 is the only diamond-chain compound composed of more than one type of dimer, namely, (FeIII)2 and (FeII)2. On the basis of temperature-dependent 57Fe Mössbauer spectroscopy data, an alternating noncollinear 90° magnetic structure is proposed. Both the (FeIII)2 and (FeII)2 dimers are antiferromagnetically coupled and align in the direction along the chain axis ≈ [010], whereas the moments of the bridging FeIII monomers are oriented orthogonally. The spin canting, arising from the anisotropy of the FeII ions, leads to ferrimagnetic ordering at low temperatures.