Conventionally, being a King, Queen, or a Princess requires you to be born as one, Royal Blood is valuable after all, and as far as most monarchs are concerned, they have to make sure their bloodline keeps going if they want their kingdoms to last.
But when you're this character, it's another story.
Maybe you got a father who was lucky enough to marry a queen, maybe you reported and helped stop a coup attempt and the royal family saw adopting you as a way of thanking you, maybe you're already of some position and are adopted into the family as a political fodder, or maybe you just befriended a prince and he liked you enough that he came to his father and begged him (if not outright demanded) you get adopted into the royal family so you and he could spend more time together. This gets an extra layer of drama when this overlaps with Oblivious Adoption - The Reveal or Internal Reveal that they are merely adopted into royalty (depending on the timing) would be a big twist for the adoptee, which will lead to many complications. However it happened, you're suddenly now at the top of the food chain of society, we hope you're comfortable (and you likely will be.)
Like adoptions everywhere else, being adopted into a royal family can have varied results ranging from being straight up Happily Adopted to their adoptive royal parents viewing them with contempt and outright abusing their adopted child. Though regardless, usually the adoptee still gets to enjoy all (or at least some of) the luxuries and privileges that go with their newfound status and tend to have similar royal duties that go with it as well. There's usually unlikely the adoptee will ever inherit the throne as their adoptive siblings are blood heirs destined for it anyway, and most royal adoptees tend not to be bothered by this and even appreciate that fact as it means they won't have to worry about the pressures of being a king or queen.
...unless a coup attempt or some other freak disaster wipes out their royal adoptive family and the following Succession Crisis leaves the entire kingdom desperate enough to put the sole-surviving adoptee on the throne, to their own surprise and possible dismay. That is, if the adoptee didn't orchestrate the disaster itself, with the intent of taking over the throne.
- Ooku: When the shogun Iemetsu dies of the red pox without descendants, his nurse/surrogate mother/regent Kasuga sends for the Child by Rape he'd produced several years before, raising her in Iemetsu's place until she can produce an heir and continue the Tokugawa bloodline. As the red pox continues to ravage Japan until one man in five is lucky to reach adulthood, the pretense of a male shogun is eventually lifted and women openly occupy government positions, while the shogun's harem is exclusively men there to guard her and father children.
- In The Seven Deadly Sins, the king of Liones adopted Elizabeth after she was carried out of the ruins of Danafor because, paradoxically, he saw that he would. Also, he just really likes being a dad. No-one's really worried about inheritance shenanigans because he has two elder daughters from his bloodline.
- In Tenchi Muyo! GXP the protagonist, Seina Yamada, an Ordinary High-School Student, gets adopted into the royal Jurai family after bonding with the Humongous Mecha holding at its heart the seed of Juraian Royal Tree — this is actually the one and only way Juraian royalty is made, as per their ancient pact with Tsunami. The royals' relatives just have a privilege in bonding. Later on, his Childhood Friend and Love Interest, Kiriko Masaki, a very distant relative of the royal family, gets an official bonding and adoption ceremony because it was decided to be politically expedient. Ironically, all this happened while the heir of the clan, Tenchi Masaki, Seina's another Childhood Friend, was still a nobody, because his grandfather and the formal Crown Prince, Yosho, was still considered officially missing.
- In the Black Panther comics, Hunter is adopted by King T'Chaka after Hunter's parents die in a plane crash in Mohannda, just north of the Wakandan border. T'Chaka treats Hunter like a son and Hunter develops a deep love of Wakanda despite many Wakandans treating him with suspicion because of his white skin which marks him as a clear foreigner. However, upon the birth of T'Challa, it becomes clear that Hunter will never ascend to the throne. Hunter is instead made the leader of Wakanda's secret police, the Hatut Zeraze, and assumes the title of White Wolf. Throughout his life, he struggles to balance his love of Wakanda with his resentment of T'Challa.
- I Am Skantarios: The Byzantine empire keeps adopting adult generals into its royal family because they never seem to produce male heirs.
- Early in The Prince of Egypt, baby Moses is found and adopted by the wife of Pharaoh Seti. It's made clear that he won't be inheriting the throne, as that's the birthright of Seti's natural-born son Rameses, but they get along so well that it's not an issue.
- Dastan, a street urchin in Prince of Persia: The Sands of Time is adopted by King Sharaman after showing courage.
- Star Wars: As shown in Revenge of the Sith, Leia Amidala Skywalker was adopted by Alderaanian Viceroy Bail Organa and his wife, Queen Breha. Leia's mother Padmé actually had been the Queen of Naboo, but that's an elected position. In the novel Leia, Princess of Alderaan, it's shown that Leia being adopted is no barricade to her inheriting the throne, and that there is even a variant of the traditional Day of Demand ceremony specifically for adopted heirs, meaning she was not the first adopted Crown Princess or Prince of Alderaan.
- In Thor, Odin adopts an infant Frost Giant (a species that were long-standing enemies to his kingdom) to be raised as his son and a prince of Asgard, apparently out of compassion for the abandoned infant and in hopes of eventually promoting peace between the two species. Unfortunately, he never bothered to tell either of his children that Loki was adopted or to teach them not to view the Frost Giants as monsters, resulting in a very unhappy revelation for the now-adult Loki.
- The Ten Commandments depicts Moses this way, adopted by Princess Bithia and being treated as a son by Pharaoh Seti. Unlike in The Prince of Egypt, however, Moses and Rameses really don't get along, at least partly because Moses is actually a contender to be Seti's heir and is frequently shown to be far more competent than Rameses, further threatening the latter's perceived birthright.
- In the Age of Fire series, the Copper is adopted into the Imperial Line of the Lavadome as Tyr FeHazathant's grandson, a position solidified by marrying the Tyr's granddaughter Halaflora. Despite this, he's so far down the potential line of succession, and is looked down on for his various personality quirks and crippling injuries, that no one ever expects him to become the Tyr himself, instead seeing the adoption as an honorary reward. However, by the end of his focus book, everyone above him in the line is either dead or in exile, allowing him to take the throne.
- The Heroic Legend of Arslan: Arslan is adopted by King Andragoras III and Queen Tahamine, something he himself doesn't even know.
- The first Safehold book, Off Armageddon Reef, ends with King Haraahld of Charis dying of a wound incurred Taking the Bullet that a nine year old midshipman had tried to take for him. To honor this act and the king's death, the boy, Hektor Aplyn, is adopted into the royal family by Haraahld's successor, Cayleb and made a Duke in the process. It's noted that Hektor Aplyn-Ahrmahk stands outside the line of succession, but his title and deeds as he grows as a naval officer make sure he can take care of his biological family while being of use to his new step-family.
- Shades of Magic: Kell was adopted by the Mareshes, the Royal family of Red London, due to being an Antari, an extremely powerful magician, so that they might manage him and use his abilities for their own gain.
- In the Temeraire books, Temeraire is of a bloodline of dragons who by tradition take as their companions only members of the Chinese royal family. Temeraire hatched from a twin egg—his brother is companion to the Crown Prince. In order to prevent Temeraire's companion from becoming a rival claimant to the throne, his egg was sent as a gift to the ruler of France, chosen for being (1) royalty and (2) very, very, very far away. When Temeraire ends up taking a non-royal person as companion, certain Chinese people are very upset. The eventual resolution is for the Chinese ruler to adopt Temeraire's companion.
- In the story of Oedipus, he was abandoned by his father due to a patricidal prophecy that ended with him marrying his mother. However, he was found and adopted into the Corinthian royal family instead. Played With in that Oedipus was actually already royalty by birth.
- The Saga of the Jomsvikings: To hush up an incestuous affair, Jarl Arnfinn of Saxony abandons his newborn son in a forest so it will be found by king Gorm of Denmark. Gorm, who at once infers from the costly garments and the gold found along with him that the boy is of noble birth, has him raised like a son and eventually names him his heir. The boy, Knut, becomes king and gives his name to the royal house of Knytlings.
- Princess: The Hopeful: While for most courts "Prince"/"Princess" is a purely honorary title, it's mentioned that all Radiants of the Court Of Hearts are actually legally adopted by the Queen of Hearts as part of an old custom from the days when Andarta was still a mass of independent city-states. 99% of the time it's irrelevant, but should you happen to run into a curse that only an actual child of a queen can break...
- A Happily Adopted case with Cecil Harvey of Final Fantasy IV as he was adopted as an orphan by the king of Baron after he found Cecil as a newborn, abandoned by his older brother Theodor due to Zemus' influence. The position he was in growing up led to him becoming childhood friends with Kain Harvey and future Love Interest Rosa Joanna Farrell. This eventually leads to him becoming the king of Baron as the former king had no actual heir when he died, leaving him essentially heir by default.
- Final Fantasy Tactics has Princess Ovelia Atkascha, who was adopted when it appeared that the King of Ivalice was too sickly to sire an heir. He did, not long before his death, but debates over whether or not the Prince was truly his son led to the outbreak of war. Ovelia herself, and the public at large, remain unaware of Ovelia's adopted status, though the Princess learns the truth not long before the war breaks out.[[/spoiler
- This happens in Lost Odyssey when Gongora takes over Uhra after killing (so the Uhrans think) Tolten. Because Tolten had reinstated the monarchy, and had no heir, Gongora was in line and became part of the royal family through a blood ceremony.
- Throughout much of the Total War franchise (particularly Rome and Medieval II), Captains are the default leader of an army when no general or royal family member is present. If they win an epic battle, they may be adopted into the ruling family. They are treated as a son and can be named Faction Heir, eventually even becoming the Faction Leader. Promoted Captains almost always come with a command rating of 3-4, sometimes come with other beneficial traits such as "Brave" and "Battle Scarred," and typically come with no negative traits. (Unlike those born into the ruling families, who are far more likely to develop negative traits.) Because of this, promoted Captains in enemy armies can be quite difficult as foes.
- Rod and Emelaigne from Cinderella Phenomenon became prince and princess in the game's backstory after their mother Ophelia (previously a baker) married Angielle's King Genaro Britton III.
- In Girl Genius, Jiminez Hoffman was adopted by the Mole King for suggesting a way for them to end their war with the Argurons: Altar Diplomacy. Which they decided he should do, since he was now a Mole Prince, and also the same species (human) as the Argurons. It turns out to be a Perfectly Arranged Marriage in the end, although it takes them most of the Paris arc before Hoffman figures it out—Princess Larana having a bad case of Cannot Spit It Out.
- My Little Pony: Friendship Is Magic: Princess Cadance, the eventual ruler of the Crystal Empire and the third alicorn introduced is not blood related to her aunts Luna and Celestia. As revealed in Twilight Sparkle and the Crystal Heart Spell, Princess Celestia adopted her niece Cadance as a foal. This explains where Cadance came from, as Celestia's only sibling has been trapped in the moon for a millenia.
- Sofia the First gives us Sofia, who was born a commoner until her shoe-maker mother married King Roland II, which in turn her a princess and her mother a queen.
- The ThunderCats (2011) version of Tygra is Lion-O's adopted older brother and Happily Adopted by King Claudus. Interestingly, King Claudus actually favors Tygra thanks to him being more of The Dutiful Son in his eyes than Lion-O is.
- The Dragon Prince: Callum became part of the royal family following his mother Sarai's marriage to King Harrow. While Soren teases him for being a "step-prince" and he is implied to harbor some insecurity regarding his relationship with Harrow, his stepfather and half-brother treat him well.
- Three of the first five Roman Emperors, and four of the "five good Emperors", were adopted as adults by their predecessors, explicitly to name them as successors.
- The Julio-Claudian Dynasty: Julius Caesar was Just the First Citizen but his adopted heir Augustus named himself the first Imperator. Augustus was succeeded by his stepson Tiberius and Tiberius adopted his great-nephew Caligula. The Praetorian Guard killed Caligula's entire family save for his uncle Claudius, whom they installed as Emperor. Claudius named his stepson Nero heir until his natural son came of age but Nero's mother assassinated him before that could happen.
- The "Five Good Emperors": Nerva was appointed by the Senate after Domitian's assassination, Nerva adopted Trajan to head off a civil war, Trajan allegedly adopted his relative Hadrian on his deathbed, Hadrian wanted Marcus Aurelius to succeed him but thought him too young so he adopted Antoninus Pius on the condition that he adopt his relative Marcus. Marcus was the second Roman Emperor to pass the throne to a natural son (after Vespasian, Domitian's father).
- Charles XIV of Sweden was actually adopted into the Swedish nobility and eventually was elected the heir to the throne.