We investigate uniaxial-stress effects on the magnetic phase diagram of the square-lattice itinerant magnet ${\mathrm{EuAl}}_4$, where strong coupling among spin, lattice, and charge produces a variety of helimagnetic phases, including rhombic and square skyrmion lattices. Combining resistivity and magnetization measurements with neutron scattering, we find that compressive stresses of only several tens of megapascal along [010] enhance antiferromagnetic character and shorten the magnetic modulation period in the lowest-temperature single-$\mathbf{Q}$ spiral state, thereby driving the critical temperatures and fields of multiple phases to higher values. First-principles calculations show that increasing orthorhombic lattice distortion deforms the Fermi surface relevant to the magnetism, providing compelling evidence that Fermi-surface nesting plays a crucial role in stabilizing the helical magnetic modulations in ${\mathrm{EuAl}}_4$.

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• Magnetic texture
• Helimagnets
• Density functional theory
• Neutron scattering