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The coupling between ferroelectric and ferroelastic order parameters in transition metal oxide systems offer unique opportunities to employ strain as a tuning parameter of polar order at the nanoscale limit with large scale technological implications. KNbO₃ is an archetypal ferroelectric with a rich structural phase diagram in the bulk and provides the possibility of stabilising non-trivial phases through strain-engineering which are otherwise thermodynamically inaccessible. The ability to grow KNbO₃ thin films by sub-oxide molecular beam epitaxy allows the application of large strains with a high degree of control in epitaxial thin films. The new stabilised structure is probed through temperature dependent second harmonic generation, synchrotron-based x-ray reciprocal space mapping and transmission electron microscopy which reveals a giant enhancement in ferroelectric Tc, polarisation and nonlinear optical coefficients even surpassing PbTiO₃ and BaTiO₃. Thermodynamic phase field simulations and density functional modeling also supports the experimentally achieved strain-phase diagram for KNbO₃.