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Nonlinear optical (NLO) switch materials that turn on/off second‐harmonic generation (SHG) at a phase transition temperature (Tc) are promising for applications in the fields of photoswitching and optical computing. However, precise control of Tc remains challenging, mainly because a linearly tunable Tc has not been reported to date. Herein, we report a unique selenate, tetragonal P4‾ ${\bar{4}}$ 21c [Ag(NH3)2]2SeO4 with a=b=8.5569(2) Å and c=6.5208(2) Å that exhibits a strong SHG intensity (1.3×KDP) and a large birefringence (Δnobv.=0.08). This compound forms a series of isostructural solid‐solution crystals [Ag(NH3)2]2SxSe1−xO4 (x=0–1.00) that exhibit excellent NLO switching performance and an unprecedented linearly tunable Tc,x,exp.=T0-kx ${{T}_{\left(c,{\rm \ }x\right),{\rm \ }\left({\rm e}{\rm x}{\rm p}.\right)}{\rm \ }={T}_{0}-kx}$ spanning 430 to 356 K. The breaking of localized hydrogen bonds between SeO42− and the cation triggers a phase transition accompanied by hydrogen bond length changes with increasing x and a linear change in the enthalpy ΔHx=ΔU1-ΔU2x+ΔU2 ${{{\rm { \Delta{}}}H}_{x}=\left({\rm { \Delta{}}}{U}_{1}-{\rm { \Delta{}}}{U}_{2}\right)x+{\rm { \Delta{}}}{U}_{2}}$. [ABSTRACT FROM AUTHOR]