The high-temperature heat treatment of electroless nickel–phosphorus (Ni-P) coatings in an air environment, and its consequences have scarcely been investigated. This work investigated tribological characteristics of the high-temperature, heat-treated, electroless Ni-P coatings on steel substrates with low-, mid-, and high-phosphorus content for which the average phosphorus content was 2.4 wt.%, 7.1 wt.%, and 10.3 wt.%, respectively. X-ray fluorescence and energy dispersive spectroscopy were implemented to determine the phosphorus content of the coatings. The oxidation of Ni and the formation of the NiO layer on the coating surface was confirmed by the X-ray diffraction technique. A reciprocating sliding method on a ball-on-flat system was utilized to evaluate the coating’s friction and wear behavior. Among the coatings with varying phosphorus content, a high hardness of 1086 HV was found for high-phosphorus coating when heat-treated at 400 °C in an air environment, and that was decreased to 691 HV when heat-treated at 650 °C. The oxidation of nickel in the electroless Ni-P coating occurred when heat-treated at 400 °C in an air environment, and this phenomenon was increased more when the temperature was increased to 650 °C. The characteristics of the NiO layer that formed on the surface of the heat-treated electroless Ni-P coating were influenced by the concentration of phosphorus, which caused different colors of NiO to be seen on the Ni-P coating surface. A greenish black NiO layer on the low-phosphorus and black NiO layer on the mid- and high-phosphorus Ni-P coating was developed during heat treatment at 650 °C in an air atmosphere. The adhesion and tribological characteristics of the Ni-P coatings were affected by the NiO layer developed on the heat-treated Ni-P coating surfaces. The Ni-P coatings with mid- and high-phosphorus content showed enhanced wear-resistance characteristics when they underwent heat treatment in an air atmosphere at the high temperature of 650 °C. The wear volume obtained for as-plated mid-phosphorus and high-phosphorus Ni-P coatings was 0.111 mm 3 and 0.128 mm 3, respectively, and that was reduced to 0.031 mm 3 and 0.051 mm 3, respectively, after the high-temperature heat treatment.