Altermagnetism has recently drawn considerable attention in three- and two-dimensional materials. Here we extend this concept to quasi-one-dimensional (Q1D) monolayers assembled from single-atomic magnetic chains. Through systematically examining nine types of structures, two stacking orders, intra- and interchain magnetic couplings, we identify four out of 30 promising structural prototypes for hosting altermagnetism, which yields 192 potential monolayer materials. We further confirm eight thermodynamically stable Q1D monolayers via high-throughput calculations. Using symmetry analysis and first-principles calculations, we find that the existence of altermagnetism is determined by the type of interchain magnetic coupling and predict three intrinsic altermagnets, ${\mathrm{CrBr}}_3$, ${\mathrm{VBr}}_3$, and ${\mathrm{MnBr}}_3$, due to their ferromagnetic interchain couplings and five extrinsic ones, ${\mathrm{CrF}}_3$, ${\mathrm{CrCl}}_3$, ${\mathrm{CrI}}_3$, ${\mathrm{FeCl}}_3$, and ${\mathrm{CoTe}}_3$, ascribed to their neglectable or antiferromagnetic interchain couplings. Moreover, the interchain magnetic coupling here is highly tunable by manipulating the interchain spacing, leading to experimentally feasible transitions between altermagnetic and nodal-line semiconducting states. In addition, applying external electric fields can further modulate the spin splitting. Our findings establish a highly tunable family of Q1D altermagnets, offering fundamental insights into the intricate relationship between geometry, electronic structure, and magnetism. These discoveries hold significant promises for experimental realization and future spintronic applications.

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• Electronic structure
• Magnetic coupling
• Magnetic interactions
• Magnetic phase transitions
• 2-dimensional systems
• First-principles calculations in plasma physics
• High-throughput calculations