A novel 3D indium metal–organic framework, namely, {[EMIM] 2 [InK(1,2,4,5-BTC) 1.5 (H 2 O) 2 ]} n (1) (1,2,4,5-BTC=1,2,4,5-benzenetetracarboxylate), has been synthesized under ionothermal conditions by using the ionic liquid 1-ethyl-3-methylimidazolium bromide ([EMIM]Br) as solvent. Two In and two K atoms were linked by two μ 3 -H 2 O to give a [In 2 K 2 (μ 3 -H 2 O) 2 ] tetranuclear cluster, which are connected by 1,2,4,5-BTC ligands to form the 3D anionic framework of compound 1. Furthermore, the [EMIM] + cations acting as extra framework charge-balancing species occupy the channels of the 3D anionic framework. Notably, compound 1 shows strong blue luminescence and remarkable photocatalytic degradation of methylthionine chloride.