The best warriors in human history are the Space Marines, strong fighters who have undergone rigorous training, genetic engineering, and psychological conditioning. Their fleets specialise in aggressive boarding tactics, have well-armed ships, and are experts at close-quarters combat. Additionally, they have access to two unique faction qualities that protect them from insubordination mechanics and let them choose which subsystems to assault first while boarding.

There are five separate Space Marine chapters in the Space Marines DLC:.

• Shadow Angels :Dark Angels get a Chapter culture that revolves around secrecy and penance because of their troubled past. Azrael, their Chapter Master, makes no compromises in his efforts to safeguard humanity, conceal the truth, and, when practical, track down the Fallen, previous Battle Brothers who abandoned the Emperor.
• Battle Angels : The Blood Angels are a Chapter renowned for their exceptional longevity, especially among Space Marines, as well as their great bloodlust and occasional lethal wrath. Dante, their more than millennia-old Chapter Master, guides his Chapter with great wisdom and effectiveness.
• Ultramarines : The Ultramarines consider themselves a model for all Space Marines Chapters because of their uncompromising commitment to the Codex Astartes. The Codex embodies rules and reverence for the order of the Ultramarines and is more than just a list of laws. They rank among the Imperium's top warriors.
• Empire Fists: The Imperial Fists are known for being one of the Emperor's most devoted Chapters. Their tenacious resiliency has been crucial in keeping the Imperium together through the darkest times. They are experts in siege warfare, and their leadership of the battle of Earth earned them the moniker "Defenders of Terra."
• Wolves of Space : During the Great Crusade, the Space Wolves gained a reputation as the Emperor's executioners and means of punishment. They are infamous for their fury and disobedience of authority, especially the rules of the Codex Astartes, throughout the remainder of the Imperium. Of all the Space Marine Chapters, they are typically regarded as having the best and most devoted warriors.

Gameplay:

The Imperial Navy, the Chaos Fleet, the Eldar Corsairs, and the Ork Freebooters are the four factions that the players command. Each party has its distinctive vessels and preferred playing style. Weapons, crew, subsystems, and Void Shields can all be modified by players for their warships.

Players can Level up using a system where they receive experience from combat. When they win, they gain more experience; if they lose, they earn less. After reaching a certain Level, players can acquire better ship classes, different upgrade kits, and crew members.

Before purchasing upgrades, players must earn notoriety points to customise their ships. Except for the Orks' ships, every ship in the Game has a set amount of space for upgrades.