jats:pThe Zintl phases Kjats:sub4–jats:italicx</jats:italic></jats:sub>Najats:italicjats:subx</jats:sub></jats:italic>Sijats:sub4</jats:sub> (1 ≤ jats:italicx</jats:italic> ≤ 2.2) and Kjats:sub7</jats:sub>NaSijats:sub8</jats:sub> are the first representatives of the K–Na–Si system, and both contain tetrahedral [Sijats:sub4</jats:sub>]jats:sup4–</jats:sup> clusters and a charge‐balancing number of Kjats:sup+</jats:sup> and Najats:sup+</jats:sup> cations. All phases of Kjats:sub4–jats:italicx</jats:italic></jats:sub>Najats:italicjats:subx</jats:sub></jats:italic>Sijats:sub4</jats:sub> (1 ≤ jats:italicx</jats:italic> ≤ 2.2) crystallize in a new structure type with space group jats:italicP</jats:italic>2jats:sub1</jats:sub>/jats:italicn</jats:italic>, as determined by single‐crystal X‐ray diffraction analysis of the parent phase Kjats:sub3</jats:sub>NaSijats:sub4</jats:sub>. Rietveld refinement of the X‐ray diffraction data showed that the solid solutions follow Vegard's rule. Kjats:sub7</jats:sub>NaSijats:sub8</jats:sub> can only be synthesized by diffusion‐controlled reaction of binary precursors and is isostructural with known jats:italicA</jats:italic>jats:sub7</jats:sub>jats:italicA′T</jats:italic>jats:sub8</jats:sub> (jats:italicA</jats:italic> = Na–Cs; jats:italicA′</jats:italic> = Li, Na; jats:italicT</jats:italic> = Si, Ge) phases. A combination of solid‐state NMR investigations and quantum mechanical calculations served to show the anisotropic chemical bonding behavior of all the atoms in Kjats:sub7</jats:sub>NaSijats:sub8</jats:sub>, which is additionally compared with the related phases jats:italicA</jats:italic>jats:sub7</jats:sub>NaSijats:sub8</jats:sub> (jats:italicA</jats:italic> = Rb, Cs).</jats:p>