<p>Lightweight compositionally complex steels (CCSs) with multiple principal elements, including Fe, Mn, Al, and C, attract attention for their excellent strength-ductility synergy at reduced density. To provide insights into the further development of lightweight CCSs with enhanced mechanical properties, we introduce a 3D-printed high-strength and ductile CCS variant decorated by segregation-dislocation cellular structures characterized by Mn segregation at dislocation-rich cell walls. The cellular structures show an average size of ~ 586&#xa0;nm, and a high dislocation density of 6.64 × 10¹⁴ m⁻² is then presented in the bulk CCS. The formation of such a unique microstructure is facilitated by the high cooling rate and thermal cycling inherent to the laser powder bed fusion. The CCS shows an excellent combination of ultimate tensile strength (1155&#xa0;MPa) and specific yield strength (132&#xa0;MPa&#xa0;cm³·g⁻¹) at a high uniform elongation (35%). Upon deformation, besides dynamic slip band refinement and stacking faulting, mechanical twinning is activated at high flow stress levels, enhancing the strain hardening at later deformation stages. This high-stress twinning behavior is unexpected since the CCS has a high nominal stacking fault energy (SFE) of 72.3&#xa0;mJ·m⁻², significantly above those for twinning in ordinary face-centered cubic alloys (e.g., &lt; 50&#xa0;mJ·m⁻²). The segregation-dislocation cellular structures promote the high-stress twinning by the synergistic effects of high flow stress arising from high-density dislocations and the reduction of local SFE caused by elemental segregation at the cell walls. These findings offer valuable insights for developing strong and ductile materials via tuning dislocation configuration and elemental segregation.</p>