Trinity Blood (Japanese: トリニティ・ブラッド, Hepburn: Toriniti Buraddo) is a series of Japanese light novels written by Sunao Yoshida with illustrations by Thores Shibamoto and originally serialized in The Sneaker. Set 900 years after an apocalyptic war between humans and vampires, the series focuses on the ongoing cold war between the Vatican, the human government, and the "New Human Empire", the government of the vampiric Methuselah. Fighting on the Vatican's side is Abel Nightroad, a Crusnik—a vampire that feeds on vampires. The novels blend science fiction, fantasy, and political intrigue, with some in the Vatican and the Empire striving for peace, and the Rosen Kreuz Order doing anything in its power to stop it.

Trinity Blood has been adapted into both manga and anime series, with continuity differences between the three versions. After Yoshida's death in 2004, his friend Kentaro Yasui aided in completing the manga, with illustrations by Kiyo Kyūjō. The anime series, spanning 24 episodes, was produced by Gonzo and originally aired on Wowow network. Both adaptations have the same unresolved conclusion as the final novel.

When Earth's population drastically increased, humanity, led by the United Nations, attempted to colonize Mars. During the colonization, they discovered two alien technologies: the Bacillus virus and the Crusnik nanomachines. The colonists injected the Bacillus in their bodies which transformed them into a vampiric race known as "Methuselah". They installed the Crusnik in the bodies of four test tube babies: Seth, Cain, Abel and Lilith whose enhanced bodies were the only ones able to survive the procedure. Continuing wars on Earth eventually led to Armageddon, the apocalyptic event that happened 900 years before the start of the story, and the colonists returned to Earth to help with rebuilding. However, when the colonists returned to Earth, a war broke out between the Methuselah and the humans who had remained on Earth. Abel, Cain, and Seth sided with the Methuselah—while Lilith supported the Vatican, which guarded humanity. During the war, Cain went insane and killed Lilith. In grief, Abel took her body to the Vatican where she was buried. Abel remained at her side, weeping for her, for 900 years.

At the start of the story, the Methuselah, still a major political and military force, continue to wage war on the "Terrans", as they call the human inhabitants of Earth. The Roman Catholic Church is a major military power determined to protect humans from the Methuselah, with its seat of power based in the Vatican. The Methuselah have their capital in Byzantium, which is surrounded by a field of particles to filter out UV radiation, protecting the Methuselah population. Both groups use "lost technologies", such as airships, missiles, and computers, to engage in a cold war with one another. A third great power, Albion also plays a role in the war with its superior arsenal of lost technology and weapons and higher level of manufacturing ability versus the Vatican. The independent monarchy of Albion is a primarily human country, however the secret of their expertise in lost technology is found in the Ghetto, an underground city of enslaved Methuselah. It is these Methuselah who operate and manufacture the lost technology, but with the death of the Albion Queen, some of the enslaved vampires begin a rebellion for the freedom of all of the Ghetto residents.

While the novels, manga, and anime series all cover the same basic story and feature many of the same characters, they do have some minor and major differences. Each has its own unique variation on the major story arcs, and in general the novels give the most detail concerning the political elements and background information on the stories. There are a few minor name changes and many details found in the novels are not given in the anime or manga.

The artwork also differs from all three versions, as the character designs were each created by different artists. There are similarities, however, as Shibamoto is responsible for the novel illustrations, while Kiyo Kyūjō based his work in the manga on Shibamoto's original designs. In the anime series, Dietrich von Lohengrin's cold, evil nature is reflected in his appearance, while in the manga he has a softer, bishōnen design that is incongruous with his actual nature. Similarly, Endre is described as looking like a boy of around 10–12 years old, while in the anime he is given an adult appearance. In additions to variants in appearances, there are differences in personalities relationships between the versions, and there are some characters that are unique to each adaptation. Sister Noelle, whose death devastates Abel in the anime series and in the novels, does not exist in the manga series. Alternatively, the anime does not mention Father Tres' two "brothers".

Depending on the adaptation, some events occur at varying points in the story, while other events are unique to a single adaptation. The anime series' introductory-style episodes are based on the novel segments with the same names, but they are told in a different story. For example, in the novels, "From the Empire", the chapter in which Abel meets Asthe is the third chapter, following Flight Night and Witch Hunt, However, in the anime, "From the Empire" is the eleventh episode, coming after the Star of Sorrow arc and the Silent Noise incident. Conversely, the manga starts right with the Star of Sorrow arc and Abel meeting Esther and Dietrich. The Neumann brothers and their related story elements are found only in the novels. In the novels, it was Abel who interrupted the Cardinal's gathering, rather than Caterina or Leon, and the only people available to investigate the Silent Noise incident in Rome are Abel, Tres, and Leon.

The humans once attempted to colonize Mars after a massive population explosion. While on Mars, they discovered the Bacillus and Crusnik nanomachines. When the Bacillus virus was embedded in humans, they were infected with a virus that changed them into vampires who had to feed on blood to survive. While this was happening on Mars, on Earth, continuing wars resulted in Armageddon, the apocalyptic war that nearly destroyed humanity. The vampires—who now call themselves Methuselahs—and the humans ended up in a war against each other when the Methuselah returned to Earth. Nine hundred years later the surviving humans are still trying to recover from the war's devastating effects while trying to find ways to co-exist with the Methuselah despite the distrust and hatred still evident on both sides. The Vatican serves as the primary leader for the people in the ongoing struggle for peace with the Methuselah, and in dealing with rogue factions from both sides that would like the war to continue. The Methuselah refer to humans as "Terrans", or the "Short-Lived Race".