We predicted highly stable Cu-decorated zigzag graphene nanoribbons (ZGNRs) containing planar tetracoordinate carbon (ptC) strip(s). The computed electronic band structures suggest that the Cu-decorated ZGNRs are semiconducting. The Hirshfeld spin analysis suggests significant spin distribution at the edge ptC atoms. The two ptC strips are antiferromagnetically coupled, even though the edge Cu atoms do not exhibit significant magnetism. The stability of the multicenter ptC strip stems from both the highly delocalized pi orbital of ZGNRs and nearly perfect match between Cu-Cu bonding geometries and carbon atoms at the ZGNR edges.