In iron-based superconductors (FeSCs), the interplay between electronic nematicity and superconductivity is essential for understanding the exotic superconducting ground state. In the nematic regime, in-plane strain ($\varepsilon$) tuning of the superconducting transition temperature $T_c [\mathrm{\Delta }{T}_c\left(\varepsilon \right)=\alpha \varepsilon +\beta {\varepsilon }^2]$ offers a unique approach to investigate the evolution of pairing symmetry if both $s$- and $d$-wave pairing instabilities are relevant. Here, we employ in-plane strain to tune the $T_c$ of ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$, in which both nematicity and superconductivity undergo significant changes with doping. While $T_c$ is usually suppressed quadratically with $\varepsilon$ in optimally doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2, \mathrm{\Delta }{T}_c\left(\varepsilon \right)$ in ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$ dominated by $\mathrm{\Delta }{T}_c\left(\varepsilon \right)=\beta {\varepsilon }^2$ changes its sign from $\beta$ < 0 in FeSe to $\beta$ > 0 in ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$ ($x\gtrsim 0.10$), indicating an evolution of the pairing symmetry from an $s_{\pm }$ state toward an $s+d$ wave state. These findings highlight the $\mathrm{\Delta }{T}_c\left(\varepsilon \right)$ as a powerful probe for elucidating the superconducting pairing symmetry in the nematic regime of FeSCs and provide unique insights into the evolution of pairing symmetry in FeSCs.

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• Multiband superconductivity
• Superconductivity
• High-temperature superconductors
• Iron-based superconductors
• Resistivity measurements
• Strain engineering