RL6137, a 'Thatcher' backcross line of wheat (Triticum aestivum L.) has resistance to Puccinia triticina (causal agent of wheat leaf rust) derived from T. monococcum. To determine the chromosome location of this resistance, RL6137 was crossed with Thatcher and F2 individual seedlings were tested for segregation of leaf rust resistance and segregation of polymorphic simple sequence repeat (SSR) markers. Leaf rust resistance genotypes of F2 individuals were confirmed with segregation of F3 families. The F2 seedlings and F3 families segregated for a single leaf rust resistance gene. The SSR markers barc 57 and barc 321 located on chromosome 3AS were tightly linked with the leaf rust resistance gene. The leaf rust resistance gene in RL6137 was designated as Lr63. This gene conditions low to intermediate infection types to most P. triticina isolates and can be used in wheat improvement programs to enhance leaf rust resistance.