The paper presents the results of studies concerning the influence of the heat input of gas metal arc welding on the microstructure and mechanical properties of the metal of the heat-affected zone (HAZ) of a high-hardness armour plate made according to MIL-A-46100. Using the bead test method, the heat input is varied in the range of 0.9-2.2 kJ$\cdot$mm$^{-1}$, and the distribution of microhardness inside the metal of the welded samples, the microstructure, and the impact toughness of the HAZ sections are investigated. The studied steel reveals a high sensitivity to welding heat. According to the microhardness distribution, the entire metal of the HAZ, the width of which increased from $\cong$ 25.0 to 28.0 mm with increasing heat input in the studied range of values, is characterized by a lower hardness compared to the base metal. As established, the lowest impact toughness is observed in the metal directly adjacent to the weld of the overheating zone, which undergoes complete austenization, grain enlargement, and further transformation of austenite with the formation of lath martensite and bainite sheaves’ mixture under the influence of the welding thermal cycle. An increase in the heat input is accompanied by a decrease in the martensitic component and, accordingly, an increase in the bainite fraction in the metal of this part of the HAZ. In turn, this leads to the impact toughness enhancement, but only for the values of welding heat input of 0.9, 1.1, and 1.3 kJ$\cdot$mm$^{-1}$. In the case of heat input of 2.2 kJ$\cdot$mm$^{-1}$, at which the metal of the overheated area underwent the most significant softening, its impact toughness was significantly lower compared to the case of welding with lower heat-input values and 30% lower than that of the base metal. This effect can be explained by a modification in the morphology of the carbide component of the bainite phase, with the gradual replacement of lower bainite by upper bainite, as well as the release of boron-containing carbide particles along the boundaries of primary grains due to excessive slowing of metal cooling in the overheated area. Thus, welding of the studied steel using parameters ensuring high deposition rate with a heat input of about 2.2 kJ$\cdot$mm$^{-1}$ can cause resistance degradation of the metal of the HAZ to brittle fractures under the influence of dynamic loads, providing a negative impact on the ballistic resistance and durability of its welded joints.