The asymmetric unit of the title complex, aqua[1‐cyclopropyl‐7‐(4‐ethylpiperazin‐1‐yl)‐6‐fluoro‐4‐oxo‐1,4‐dihydroquinoline‐3‐carboxylato](1,10‐phenanthroline)copper(II) nitrate dihydrate, [Cu(C19H21FN3O3)(C12H8N2)(H2O)]NO3·2H2O, contains one [Cu(erx)(phen)(H2O)]+ cation, one NO3− anion and two solvent water molecules (erx is enrofloxacin and phen is 1,10‐phenanthroline). The CuII ion adopts a slightly distorted square‐pyramidal geometry, coordinated by two O atoms of the erx ligand and two N atoms of the phen ligand in the basal plane, and by a water molecule in the apical position. The CuII ion deviates from the basal plane by 0.1439 (7) Å. The piperazine ring belonging to erx has a slightly twisted chair conformation. Coordinated and uncoordinated water molecules participate in a hydrogen‐bonding network including both cations and anions, to produce a three‐dimensional supramolecular structure.