[en] The branching ratio for the superallowed β⁺ decay of ³⁸K^m was measured at TRIUMF's ISAC radioactive ion beam facility. The M3 internal transition between the isomer and the ground state of ³⁸K^m was observed with a branching ratio of 330(43) ppm. A search for the nonanalogue β-decay branch to the first excited 0⁺ state in ³⁸Ar was also performed and yielded an upper limit of ≤12 ppm at 90% C.L. These measurements lead to a revised superallowed branching ratio for ³⁸K^m of 99.967(4)%, and increase the ³⁸K^m ft value by its entire quoted uncertainty to ft=3052.1(10) s. Implications for tests of the nuclear-structure dependent corrections in superallowed β decays and the extraction of the Cabibbo-Kobayashi-Maskawa matrix element V_ud are discussed