<p>Nowadays, high-strength steels are becoming increasingly important because of their beneficial properties and versatility. Due to advanced manufacturing technologies, steels with yield strengths of up to 1300&#xa0;MPa can be produced for thick plates, although their application remains limited. High-strength steels are usually joined by welding, but the strength of the filler metals may be lower than that of the steels themselves. Above 1100&#xa0;MPa of yield strength, there is no matching type of filler metal so that matching filler metals cannot always be used. High-strength steels are also often subjected to cyclic loading, which can lead to fatigue failure of structural elements or entire structures. In this research, to investigate the effect of the filler metal, different high-strength structural steels (with varying strengths and manufacturing methods) from the 700–1300&#xa0;MPa strength category were welded using different filler metals from various strength categories, and fatigue crack propagation tests were performed. In addition, on the different base materials, fatigue crack propagation tests were also carried out. In order to obtain more comprehensive results, the position and orientation of the notches were also varied. After the tests, the two parameters (<i>C</i> and <i>n</i>) of the Paris–Erdogan equation were determined for each specimen using the least squares regression method. The results were compared for the different cases based on the Paris–Erdogan exponents, applying statistical methods. According to the Paris–Erdogan exponent and constant, the relationship was found to be close and reliable for both the base materials and welded joints. The tested steels and their welded joints behaved differently in the longitudinal or transversal directions compared to the thickness direction. The mismatch effect was significant in the investigated directions.</p>