Using X-ray analysis, the crystal structure of yegorovite Na₄[Si₄O₈(OH)₄] · 7H₂O, a newly-discovered mineral from the Lovozero alkaline complex (Kola Peninsula, Russia), was determined. The mineral is monoclinic, of P2₁/c, a = 9.8744(4), b = 12.3981(5), c = 14.8973(7) Å, β = 104.675(5)°, V =1764.29(13)ų, Z = 4. Yegorovite is a representative of a new structure type. Its structure is based upon the zigzag chains [Si₄O₈(OH)₄]_∞ extended along [100]. The Na atoms occupy four nonequivalent crystallographic positions and are located in six-fold polyhedra [NaO(OH)₂(H₂O)₃] and [NaO(OH)(H₂O)₄]. The Na polyhedra are joined with each other by vertices and edges to form corrugated layers parallel to (001). To each of these layers, the silicon-oxygen chains adjoin from both sides, and the neighboring Na-Si layers are combined with one another by a system of H-bonds.