The ground state of a molecular diamond-lattice compound (ET)Ag 4 (CN) 5 is investigated by the magnetization and nuclear magnetic resonance spectroscopy. We found that the system exhibits antiferromagnetic long-range ordering with weak ferromagnetism at a high temperature of 102 K owing to the strong electron correlation. The spin susceptibility is well fitted into the diamond-lattice Heisenberg model with a nearest neighbor exchange coupling of 230 K, indicating the less frustrated interactions. The transition temperature elevates up to ∼195 K by applying pressure of 2 GPa, which records the highest temperature among organic molecular magnets. The first-principles band calculation suggests that the system is accessible to a three-dimensional topological semimetal with nodal Dirac lines, which has been extensively searched for a half-filling diamond lattice.A half-filling diamond lattice has been recently attracted great interests as an example of three-dimensional (3D) Dirac semimetal with the linearly-crossing band dispersion near the Fermi level [1-4] along with the appealing example including Na 3 Bi and Cd 3 As 2 [5,6]. The system can be a strong topological insulator in the presence of spin-orbit coupling as a 3D analogue to graphene [1]. Despite the popular crystal structure, the material with the half-filled band has been known only in a putative material BiO 2 [3]. In the counterpart insulating system, the frustrated local moment on the diamond lattice has been extensively studied as a spin liquid candidate [7][8][9]. A typical example is the magnetic spinel (AB 2 C 4 ) with the A site diamond lattice [10][11][12][13][14][15][16][17][18], where the properties of the disordered state are under intense debate. A (topological) Mott transition is expected to occur from a spin disordered phase to a Dirac semimetal phase by tuning the electron correlation [2,7].Organic molecular compounds have provided the platform for investigating the pressure-tuned Mott transition for the soft crystal. The well-studied Mott-Hubbard systems such as κ-(ET) 2 X and Z[Pd(dmit) 2 ] 2 possess a quasi-two-dimensional triangular lattice of the molecular dimer unit [19][20][21] owing to the anisotropic intermolecular interactions between planar molecules. Thus there are only a few example of 3D molecular compounds including the diamond lattice, except for the inorganic-organic hybrid system such as Li(TCNE) and Cu(DCNQI) 2 [22][23][24], and no example is known for the half-filling diamond lattice consisting of organic molecules. The material search for 3D molecular compounds would be important for giving high-temperature magnets and superconductors.We present here the molecular material (ET)Ag 4 (CN) 5 [25] as a prime example of the 3D diamond lattice. It possesses the extremely high-symmetry crystal structure of the orthorhombic Fddd lattice with the lattice constants: a = 13.215(9) Å, b = 19.4783(1) Å, and c = 19.6506(1) Å. Each monovalent ET molecule is surrounded by the honeycomb framework of the closed-shell polyanion [Ag 4 (C...