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The asymmetric unit of the title salt, K+·C13H14BF3NO2·0.5H2O, consists of two derivatized indolyltrifluoridoborate anions, two potassium cations and one water molecule. Within the indolyltrifluoroborate anions, the least-square planes consisting of the carboxyl group and the adjacent quarternary C atom of the tert-butyl groups deviate significantly from coplanarity with the indolyl planes [20.44 (11) and 21.02 (10)°]. The potassium ions are coordinated by six atoms (one K+ ion by two O and four F atoms, and the second K+ ion by one O and five F atoms), however, one of the potassium ions undergoes an additional weak potassium–π interaction (K...centroid = 3.722 Å). The packing is stabilized by sequential O—H...O hydrogen bonds along [100] between water molecules and also by O—H...F hydrogen bonds.