Pairing symmetry which characterizes the superconducting pairing mechanism is normally determined by measuring the superconducting gap structure ($|{\mathrm{\Delta }}_k|$). Here, we report the measurement of a strain-induced gap modulation ($\partial |{\mathrm{\Delta }}_k|$) in uniaxially strained ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_2{\mathrm{As}}_2$ utilizing angle-resolved photoemission spectroscopy and in situ strain tuning. We found that the uniaxial strain drives ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_2{\mathrm{As}}_2$ into a nematic superconducting state which breaks the fourfold rotational symmetry of the superconducting pairing. The superconducting gap increases on the $d_{yz}$ electron and hole pockets while it decreases on the $d_{xz}$ counterparts. Such orbital selectivity indicates that orbital-selective pairing exists intrinsically in non-nematic iron-based superconductors. The $d_{xz}$ and $d_{yz}$ pairing channels are balanced originally in the pristine superconducting state, but become imbalanced under uniaxial strain. Our results highlight the important role of intraorbital scattering in mediating the superconducting pairing in iron-based superconductors. It also highlights the measurement of $\partial |{\mathrm{\Delta }}_k|$ as an effective way to characterize the superconducting pairing from a perturbation perspective.

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• Superconducting gap
• Iron-based superconductors
• Angle-resolved photoemission spectroscopy