A novel molecule-based magnetic polymer Mn(N 3 ) 2 (btr) 2 1 (btr ) 4,4′-bi-1,2,4-triazole) was synthesized and characterized crystallographically and magnetically. 1 crystallizes in the monoclinic system, space group P2 1 /c, formula C 8 H 8 N 18 Mn, with a ) 12.2831(4) Å, b ) 6.3680(1) Å, c ) 10.2245(3) Å, β ) 105.064(1)°, and Z ) 2. Bridged by end-to-end azides, the Mn 2+ ions form a two-dimensional layer with (4,4) topology; the layers are further connected to the three-dimensional network by the weak hydrogen bonds between ligands of btr. Magnetic studies on a polycrystalline sample show the existence of strong antiferromagnetic couplings between the adjacent Mn 2+ ions, and the Nee ´l temperature is T N ) 23.7 K. In the ordered state below T N , detailed investigations on an oriented single-crystal sample of 1 reveal that the hidden spin-canting, metamagnetic transition, and spin-flop transition can appear in different circumstances. The ground state is of an antiferromagnet with hidden spin-canting. An external field applied along the b direction parallel to the manganese azide layer can lead to a first-order metamagnetic phase transition, while a spin-flop transition may occur when the field is applied along the a* direction that is perpendicular to the manganese azide layer. Magnetic phase diagrams in both the T-H a * and the T-H b planes were determined. Possible spin configurations before and after the transitions were proposed. Analyses on the experimental data give the following intrinsic parameters: the intra-and interlayer coupling J ≈ -3.5 cm -1 and J a * ) 6 × 10 -4 cm -1 , the anisotropy field H A ) 0.2 kOe, the exchange field H E ) 387.8 kOe, and the anisotropy parameter R ) 5 × 10 -4 . The small J a * and R show 1 to be a good example of a two-dimensional Heisenberg system.