The search for a giant Nernst effect beyond conventional mechanisms offers advantages for developing advanced thermoelectric devices and understanding charge-entropy conversion. Here, we study the Seebeck and Nernst effects in ${\mathrm{HfTe}}_5$ across a broad range of magnetic fields. Remarkably, the Nernst effect forms a giant plateau at ultrahigh magnetic fields ($B>10B_{QL}$), with the magnitude reaching up to $50\mathrm{\mu }\mathrm{V}/\mathrm{K}$ at 2 K. By tracking two magnetic-field-driven phase transitions predicted for weak topological insulators, we find that the giant Nernst plateau exists exclusively in the ideal 1D Weyl phase. Theoretical analysis further demonstrates that such a giant Nernst plateau arises from the unique thermoelectric conversion mechanism inherent to the ideal 1D Weyl phase, where the transverse thermoelectric effect (Nernst effect) is dominated by the longitudinal conduction channel. Our findings expand the understanding of ideal Weyl physics and open new avenues for significantly improving thermoelectric conversion efficiency.

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• Nernst effect
• Seebeck effect
• Thermoelectric effects
• Topological materials