BiFe(1-x)Co(x)O3 and Bi0.9Sm0.1Fe(1-x)Co(x)O3 were synthesized under a high pressure of 4 GPa; 10% Sm substitution for Bi in BiFe(1-x)Co(x)O3 (x ≤ 0.20) drastically destabilized the ferroelectric BiFeO3-type structure and changed it to an antiferroelectric PbZrO3-type superstructure. In comparison, a ferroelectric BiCoO3-type tetragonal structure (x ≥ 0.40) was insensitive to the Sm substitution. No decrease in the ferroelectric Curie temperature (TC) was observed. Weak ferromagnetism with a spontaneous moment of 0.025 μB/formula unit (f.u.) was observed for BiFe(1-x)Co(x)O3 (x = 0.10 and 0.20) samples, suggesting the change in the spin structure from a cycloidal one. Because of the coexistence of ferroelectricity and ferromagnetism at room temperature, this compound is a promising multiferroic material.