Three new antimonide Zintl phases, RbGaSb2, CsGaSb2, and CsInSb2, were discovered during exploration of corresponding A-M-Sb (A = Rb, Cs; M = Ga, In) ternary systems while searching for new clathrates. The AGaSb2 phases crystallize in the tetragonal space group P42/nmc (No. 137) in the LiBS2 structure type, while CsInSb2 crystallizes in lower symmetry in the orthorhombic space group Cmce (No. 64) in the KGaSb2 structure type with additional disorder of one of the Cs sites. The crystal structures of all three reported AMSb2 compounds are composed of two-dimensional [MSb2]- tetrahedral layers separated by Rb+ or Cs+ cations. [MSb2]- layers are built from fused M-Sb pentagons and hexagons, which are also the main structural units for A8M27Sb19 clathrate cages. The semiconductor nature of AMSb2 was suggested by band structure calculations and confirmed by transport property characterization. CsGaSb2 is a rare example of an n-type pnictide Zintl phase. All reported compounds exhibit low thermal conductivity typical for complex antimonides of heavy elements.