Three new antimonide Zintl phases, RbGaSb, CsGaSb, and CsInSb, were discovered during exploration of corresponding A–M–Sb (A = Rb, Cs; M = Ga, In) ternary systems while searching for new clathrates. The AGaSb phases crystallize in the tetragonal space group $P_2nmc$ (No. 137) in the LiBS structure type, while CsInSb crystallizes in lower symmetry in the orthorhombic space group Cmce (No. 64) in the KGaSb structure type with additional disorder of one of the Cs sites. The crystal structures of all three reported AMSb compounds are composed of two-dimensional [MSb]− tetrahedral layers separated by Rb or Cs cations. [MSb]− 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 AMSb was suggested by band structure calculations and confirmed by transport property characterization. CsGaSb 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.