This paper proposes La1−xKxFeO3 prepared by self-propagating high-temperature synthesis (SHS) as an alternative to platinum catalysts for promoting diesel soot combustion. The catalytic property of eleven products SHSed with different substitution ratios of potassium (x = 0–1) was experimentally evaluated using a thermobalance. In the mass loss curves of the product, T50 was defined as the temperature at which the weight of the reference soot decreases to half its initial weight. The BET specific surface area of SHSed La1−xKxFeO3 depended on x strongly. All the products showed good oxidation catalytic activity. Despite having the smallest surface area (0.11 m2/g) among the obtained products, La0.9K0.1FeO3 (x = 0.1) was found to be the best catalyst with the lowest T50 (442 °C). T50 of La1−xKxFeO3 decreased with increasing x for x > 0.2. The products with x = 0.6 and 0.8 were the second-best catalysts in terms of their T50. Moreover, average apparent activation energy of La0.9K0.1FeO3 (x = 0.1) calculated by Friedman method using TG was as much as 61 kJ/mol lower than that of Pt/Al2O3 catalyst. In conclusion, potassium-substituted SHSed La1−xKxFeO3 can be used as an alternative to Pt/Al2O3 for soot combustion.