Electrospark deposition (ESD), an additive microweld-surfacing process, is typically used as a new or repair technology to enhance wear and corrosion resistance for build-up and special surface modifications. ESD employs short-duration, high-current electrical pulses that result in a fused metallurgical bond to the substrate. The pulse microsecond duration splat transfer produces micro- and nanostructures and a very narrow heat-affected zone. The limited corrosion and hydrogen-induced cracking resistance of conventional ultrahigh strength 4340 type steel, greater than 1500-MPa yield strength, led to the development of precipitation-hardened, corrosion-resistant, ultrahigh-strength steel alloys such as Custom 465 and Ferrium S53. Additive surface repair technologies of these alloys in the heat-treated condition are very limited. Preheating requirements, distortion, heat-affected zones, and post-additive heat-treatment requirements prevent surface treatments in the heat-treated condition. This paper characterizes ESD applied to ultrahigh-strength steel with respect to the heat-affected zone and the deposit–substrate mixed zone through micro- and nanohardness measurements and through microstructural and microchemical characterization. The additive alloy was Stellite 21, a cobalt-based, high-chromium alloy capable of developing high hardness and high corrosion resistance. Results show that, properly applied, the heat-affected zone width ranged from 8 to 15 μm. Microhardness analysis revealed a drop in hardness within this zone. The weld dilution zone width in the deposit is on the order of 8 to 10 μm within the first two deposit layers.