Dragon Quest IX: Sentinels of the Starry Skies is a 2009 role-playing video game developed by Level-5 and Square Enix for the Nintendo DS. Published by Square Enix in Japan in July 2009 and by Nintendo overseas the following year, it is the ninth mainline entry in the Dragon Quest series. The storyline follows the protagonist, a member of the angelic Celestrian race, after a disaster in their home scatters magical fruits across the mortal realm. While carrying over traditional gameplay from the rest of the series with turn-based battles, the game is the first Dragon Quest entry to feature a customizable player character, and the first to include a multiplayer mode, with the option of trading treasure maps and loaning player characters through Nintendo Wi-Fi. Online functions ended in 2014 when it ceased operations.

Development began in 2005, with Level-5's Akihiro Hino both acting as co-director and encouraging the game's development for the DS. Series creator Yuji Horii acted as game and story designer, with artist Akira Toriyama and composer Koichi Sugiyama also returning from previous entries. While an early prototype used an action-based combat system, negative fan feedback and internal testing prompted a return to the turn-based gameplay of earlier entries. Due to the focus on multiplayer, the games narrative and gameplay were designed around these features. Some of the game design drew inspiration from The Elder Scrolls IV: Oblivion and Diablo.

Announced in 2006 with a planned release the following year, the game was delayed by two years, with the final delay to allow bug fixes prompting Square Enix to cut its profit forecasts. Localization was handled by PlusAlpha and Shloc, who had previously localized Dragon Quest VIII. Upon release in Japan, the game posted strong sales, and with over five million copies sold worldwide by 2011 was the best-selling entry in the series until the multiplatform Dragon Quest XI (2017). Reviews in both Japan and the West were generally positive, with most of the praise being directed towards its gameplay design and graphics. Several critics felt that its traditional design restricted or undermined its other elements. Hugely popular in Japan, the game's anonymous communication mode would inspire the Nintendo 3DS's in-built SpotPass and StreetPass.

Dragon Quest IX: Sentinels of the Starry Skies is a role-playing video game set in a fantasy world; the player takes on the role of a customizable player character of the Celestrian people trapped in the mortal world. The display is split between the two screens of the Nintendo DS; the top screen displays the map area and the player's location on it together with party status, while the bottom screen shows the immediate environment and main menu display. The in-game environment is displayed from an overhead isometric perspective, with both the environment and character models being displayed with 3D graphics. In addition to the main story quests, the player can pick up optional side-quests from non-player characters (NPCs) found through the world, with some being standalone and others being related chains of quests. Multiple quests can be taken on and completed at once, and can vary in content from errand tasks to defeating specific monsters. The player can use an alchemy pot to synthesise equipment, items, and weapons using ingredients gathered by foraging or finding them in treasure chests or getting them as drops from defeated monsters. Saving can only be performed at a church in town.

As with all previous mainline Dragon Quest games, Dragon Quest IX uses a traditional turn-based battle system, though enemies are displayed as roaming sprites rather than previously established random encounters. Coming into contact with an enemy sprite during exploration triggers a battle in a combat arena themed after the current location, and depending on the party's level an enemy may give chase or run away. In addition to the protagonist, the player needs to create the remaining three members. The party is restricted to four including the protagonist, with a further eight able to be held in reserve. All party members can be individually controlled or act automatically based on preset commands managed in the battle menu. The player can attack, defend, use an item, use skills dubbed Abilities, perform magic which draws on a magic resource called MP, trigger a special action called a Coup de Grâce, or attempt to escape. Consecutive normal attacks trigger a damage multiplier which remains until the sequence is broken by an enemy or a non-attack skill.

A core part of organizing the player party is assigning character classes called Vocations. The player character starts by default as a Minstrel, with access to defensive, offensive, and supportive skills, and they can later branch into other Vocations, including Warrior, Thief, and Priest. Vocations are permanently assigned to other characters, with the protagonist able to change Vocations at an in-game location after a certain point in the narrative. By using Vocations, the player can learn skills exclusive to each one. Changing Vocation resets the character's experience level to one and removes all learned spells, with skill point abilities purchased by the player along with unspent skill points being carried over, but switching back to a class restores that character's level and spells. At the end of battle, the party is awarded experience points which raise a character's level, skill points to raise specific character statistics, and gold to spend at shops in towns. If the party falls in battle, they are returned to their last save and lose half the gold they hold.

In addition to single-player gameplay, Dragon Quest IX incorporates multiplayer. Up to three designated players can connect locally, with the one who initiated the session acting as host. The party can travel together, or the four can split up and act independently. Guest party members cannot progress the narrative, but are allowed to keep any items, experience points, and gold they acquire. Guests are not restricted by level, allowing a party composition of any level to take part in quests at different parts of the story. After completing a story quest early in the game, the player could engage in a form of passive online interaction dubbed Tag Mode. While the DS is in sleep mode, it can interact with other nearby systems with the game installed, allowing up to three characters to be imported into the game bringing a gift, ranging from items and equipment to treasure maps leading to caves that can hold treasure or dungeons to explore. The online functions were not region locked, allowing trading between systems in Japan, North America, and Europe. While much of the text went untranslated, map titles were automatically translated into the DS system's native language. The online elements lasted until 2014, when the Nintendo Wi-Fi service was shut down.

In the world of Dragon Quest IX, an angelic race known as Celestrians act as guardians of the Protectorate, the realm of mortals. The purpose of aiding and protecting them is to collect the mortals' spiritual energy known as benevolessence and feed it to the World Tree Yggdrasil, located in the Celestrians' Observatory. Once fed the appropriate amount, Yggdrasil produces a fruit called Fyggs that summons a magical train dubbed the Starflight Express allowing the Celestrians to enter the Realm of the Almighty. The protagonist is a Celestrian assigned to the human village of Angel Falls under the stewardship of Aquila, gathering the last piece of benevolessence to grow the Fyggs. As they board the Starflight Express, a malevolent force attacks from below, sending the Fyggs, the protagonist, and Starflight Express down to the Protectorate. The protagonist awakens as a mortal but is still able to see Celestrians and the spirits of the dead, and is on a mission to help Angel Falls discover the broken-down Starflight Express and its fairy engineer, Stella. Stella promises to help the protagonist return to the Observatory if they can prove they are a Celestrian by gathering benevolessence. Convincing Stella of their identity, the protagonist receives a vision inspiring them to explore the Protectorate and recover the scattered Fyggs.