Graphene nanostructures with π magnetism offer a chemically tunable platform to explore quantum magnetic interactions. However, the realization of nanographene-based chains bearing controlled spin order is highly challenging due to the limited availability of building blocks and the lack of efficient polymerization strategies for chain growth. Here, we demonstrate the successful on-surface synthesis of spin-1/2 antiferromagnetic Heisenberg chains with parity-dependent magnetization based on antiaromatic diaza-hexa-peri-hexabenzocoronene (diaza-HBC) units. Using distinct synthetic strategies, two types of spin chain with different terminals are synthesized, both exhibiting a robust odd–even effect on the spin coupling along the chain. Combined investigations using scanning tunnelling microscopy, non-contact atomic force microscopy, density functional theory calculations and quantum spin models confirm the structures of the diaza-HBC chains and elucidate their magnetic properties, which have an S = 1/2 spin per unit through electron donation from the diaza-HBC core to the Au(111) substrate. Gapped excitations are observed in even-numbered chains, while enhanced Kondo resonance emerges in odd-numbered units of odd-numbered chains due to the redistribution of the unpaired spin along the chain. Our findings provide an effective strategy to construct inaccessible nanographene-based spin chains and unveil the odd–even effect in their magnetic properties, offering potential applications in nanoscale spintronics. (Figure presented.)