Two mercury selenite compounds HgM(SeO3)2(H2O)2 (M = Co, Ni) are synthesized using a conventional hydrothermal method. Both compounds crystallize in the monoclinic structure with a C2/ c space group, and equivalent Co2+ or Ni2+ ions exhibit a stacked triangular lattice. Magnetic measurements suggest different magnetic properties between these two isostructural compounds, in which HgCo(SeO3)2(H2O)2 undergoes a canted antiferromagnetic order below TN = 7.6 K, while HgNi(SeO3)2(H2O)2 exhibits a collinear antiferromagnetic order below TN = 9.0 K. When applying a magnetic field, two successive magnetic transitions are observed at Bc1 ∼ 1 T and Bc2 ∼ 3.6 T in HgCo(SeO3)2(H2O)2, whereas only one field-induced transition is detected around 3.1 T in HgNi(SeO3)2(H2O)2.