The quasi-one-dimensional (Q1D) pi-d hybrid molecular conductor (DIETSe)(2)FeBr4 undergoes an antiferromagnetic (AF) transition of d-electron spins, associated with a metal-insulator (M-I) transition at 7K. The existence of a Q1D Fermi surface in the high-temperature metallic state is confirmed by the appearance of Lebed resonances, which disappear below 7K, suggesting an AF-order-induced spin density wave transition. The application of hydrostatic pressure increases the M-I transition temperature, indicating the enhancement of the pi-d interaction. Angle-dependent magnetic torque measurements reveal a spin-flop transition of AF d-electron spins at 3.4 T. The magnetic easy axis is tilted +/- 54 deg from the b-axis towards the c-axis. The magnetoresistance (MR) shows an anomaly in the spin-flop field, associated with a small hysteresis. The AF order is suppressed with high magnetic fields on the order of 20 T, above which the MR shows a very sharp and large anomaly. A large hysteresis loop appears in the MR below the critical field of the AF transition. The results indicate a significant interplay between itinerant pi-electrons and local moments of d-electron spins.