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127I nuclear quadrupole resonance (NQR) spectroscopy is established as a rapid and robust method to indicate the formation of iodine–nitrogen halogen bonds in co‐crystalline powders. Once the relevant spectral frequency range has been established, diagnostic 127I NQR spectra can be acquired in seconds. The method is demonstrated for a series of co‐crystals of 1,4‐diiodobenzene. Changes in the 127I quadrupolar coupling constant (CQ) by up to 74.4 MHz correlate with the length of the C−I donor covalent bond and inversely with the I⋅⋅⋅N halogen‐bond length. The predictive power of this technique is validated on two previously unknown co‐crystalline powders prepared mechanochemically. Single‐crystal growth via co‐sublimation and structure determination by single‐crystal X‐ray diffraction cross‐validates the findings. Natural localized molecular‐orbital analyses provide insight into the origins of the quadrupolar coupling constants. [ABSTRACT FROM AUTHOR]