In many-electron systems, the interelectronic angle density A(theta(12)) represents the probability density function for the interelectronic angle theta(ij) spanned by position vectors r(i) and r(j) of any two electrons to be theta(12). It is shown that in general the interelectronic angle density A(theta(12)) is expressed by a linear combination of Legendre polynomials P-n(cos theta(12)). Explicit formulas for the expansion coefficients are presented for atoms described by determinantal wave functions. The results are applied to the 102 atoms He through Lr in their ground states within the Hartree-Fock framework, and the expansion coefficients as well as the average interelectronic angles are reported.