Abstract A novel metal‐organic framework based on trigonal prismatic building blocks has been hydrothermally prepared and structurally characterized. It crystallizes in the cubic space group Pa $\bar{3}$ with Z = 8. The oxo‐centered trinuclear complex units are bridged by 1,3‐benzenedicarboxylate (1,3‐BDC) ligands, forming a new 3D open‐framework [Fe 3 O(H 2 O) 3 (1,3‐BDC) 3 ][Cl]·9H 2 O ( 1 ). Its magnetic behavior has been quantitively analyzed by the Bleaney–Bowers equation, indicating the presence of antiferromagnetic interactions within the trinuclear Fe III atoms.