Ultimate focusing of an X‐ray free‐electron laser (XFEL) enables the generation of ultrahigh‐intensity X‐ray pulses. Although sub‐10 nm focusing has already been achieved using synchrotron light sources, the sub‐10 nm focusing of XFEL beams remains difficult mainly because the insufficient stability of the light source hinders the evaluation of a focused beam profile. This problem is specifically disadvantageous for the Kirkpatrick–Baez (KB) mirror focusing system, in which a slight misalignment of ∼300 nrad can degrade the focused beam. In this work, an X‐ray nanobeam of a free‐electron laser was generated using reflective KB focusing optics combined with speckle interferometry. The speckle profiles generated by 2 nm platinum particles were systematically investigated on a single‐shot basis by changing the alignment of the multilayer KB mirror system installed at the SPring‐8 Angstrom Compact Free‐Electron Laser, in combination with computer simulations. It was verified that the KB mirror alignments were optimized with the required accuracy, and a focused vertical beam of 5.8 nm (±1.2 nm) was achieved after optimization. The speckle interferometry reported in this study is expected to be an effective tool for optimizing the alignment of nano‐focusing systems and for generating an unprecedented intensity of up to 1022 W cm−2 using XFEL sources. Focusing of an X‐ray free‐electron laser enables the production of ultrahigh‐intensity X‐ray pulses. X‐ray nanobeams of a free‐electron laser were generated using reflective focusing optics combined with speckle interferometry.