The ternary lanthanum sulfide selenide β-LaS1.86(1)Se 0.14(1) was obtained by reaction of the elements in an NaCl flux. The new compound adopts the β-LnS2 structure type and crystallizes in the orthorhombic space group Pnma (no. 62) with lattice parameters of a = 814.77(1), b = 1638.46(1) and c = 413.88(1) pm. Raman lines indicate the presence of mixed (S1-ySey)2- dianions with y ≈ 0.14, besides the well known S22- dianions. The band gap of β-LaS1.86(1)Se0.14(1) is 2.5 eV as determined by optical spectroscopy.