Kingdom Hearts has many games. Almost all of them have just become available on Steam. In which order should we play them?

The first thing you should know is: the chronological order is probably not the best. By chronological order I mean the order the events take place in the series's lore. Playing in chronological order would mean playing prequels first, for example. However, prequels have a narrative function that is almost like answering a question: what happened before the story? But it doesn't make a lot of sense to have an answer for a question you didn't make, right?

The second thing you should know is: spin-offs are absolutely relevant for the story and the character development. Of course, feel free to play only the games you find interesting, but if you're invested in the story and its characters you will be missing a lot if you only play the 3 numbered Kingdom Hearts games.

1. Kingdom Hearts
Available on the 1.5 + 2.5 ReMIX collection
There's no way to go wrong here, right? Start by the beginning of Sora's journey!

2. Kingdom Hearts Re:Chain of Memories
Available on the 1.5 + 2.5 ReMIX collection
This game can be repetitive as many parts retell the first game, but its original story parts connect the first and the second game with many important plot points, including the first appearance of an important recurrent group of characters.

3. Kingdom Hearts II
Available on the 1.5 + 2.5 ReMIX collection

4. Kingdom Hearts 358/2 Days
Playable on Nintendo DS
Watchable on the 1.5 + 2.5 ReMIX collection
Here's where things start to get a bit confusing. This game was initially released for the DS, but unfortunately it wasn't completely remade for modern platforms, so the gameplay parts were eliminated. This collection only includes the remastered cutscenes and stories. This is actually a very interesting game, so if you can please play the actual game.
This game takes place at the same time as parts of Kingdom Hearts II, and some people would say to play this one first. However I think it has more impact as an exploration of characters first introduced in Kingdom Hearts II (and it was actually released after II).

5. Kingdom Hearts Birth by Sleep
Available on the 1.5 + 2.5 ReMIX collection
Our first prequel, taking place years before the first game. Again, I highly recommend to not play it in chronological order!
In this game you can choose a main character and follow their storyline. The recommended order goes:
Terra → Ventus → Aqua

6. Kingdom Hearts Re:Coded
Playable on Nintendo DS
Watchable on the 1.5 + 2.5 ReMIX collection
Like 358/2 Days, this is a DS game that wasn't fully remade for the collection, having only its cutscenes and stories remastered. Its gameplay is enjoyable for those who want to play it, but like Re:Chain of Memories most of its gameplay can be repetitive as it's also a retelling of the first game.

7. Kingdom Hearts Dream Drop Distance
Available on the 2.8 Final Chapter Prologue collection

8. Kingdom Hearts χ
Watchable online
Now things get interesting. χ was a browser game released only in Japan back in 2013. It's fine if you're not invested enough in the series to care about it, but its story is actually relevant for the Kingdom Hearts lore. Characters introduced here have appeared in the more recent games, and seem to become more and more relevant. You can watch the translated videos for the full game on YouTube (first story video below).

9. Kingdom Hearts χ Back Cover
Watchable on the 2.8 Final Chapter Prologue collection
This is an animated movie that could be seen as a retelling of the game above. That might sound great, since it'd mean you don't need to watch χ stories. The problem, however, is that χ Back Cover is more of a retelling of certain parts of the story from certain points of view. It complements χ's story, instead of replacing. So to fully get the whole story you'd need to watch both.

10. Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage-
Available on the 2.8 Final Chapter Prologue collection
A sequel to the prequel, connecting it to III.

11. Kingdom Hearts Union χ
Watchable online
Playable on smartphones
The names are similar, but Union χ is not the same game nor does it have the same story as χ. Union χ was a smartphone game that was active until 2021 and, like χ before, has a story that keeps showing itself to be more and more relevant to the series's lore. Like χ, you can also watch it online. If you're inclined to it, an offline version is available for smartphones with no microtransactions.
Quests 861 to 919 were released after Kingdom Hearts III's release, and quests 920 to 979 were released after the Re Mind DLC's release, so it'd be advised to only watch them after those.
(don't be scared by these numbers, most quests didn't have cutscenes. The actual number of cutscenes is actually closer to 120)

12. Kingdom Hearts III
Available on Steam
Includes the Re Mind DLC, which includes new stories.

13. Kingdom Hearts Dark Road
Watchable online
Playable on smartphones
Another smartphone game, with cutscenes that can be watched online. Like Union χ, it can also be played offline with no microtransactions. Like Union χ, its story is showing itself to be relevant for future games.

14. Kingdom Hearts Melody of Memory
Available on Nintendo Switch and other platforms
The series's first rhythm game. It's actually quite fun and the songs are iconic. There is only about half an hour of new story content, so if you're not inclined to play it, it's easy to find the cutscenes on YouTube.

Kingdom Hearts Missing-Link
A new smartphone game that is expected to be released in 2024. If you've read this guide, you know that smartphone games tend to be quite relevant to the series's lore, so at the very least we'll be watching its cutscenes.

Kingdom Hearts IV
WIth the Dark Seeker Saga being finished, everybody is eager to see what's coming next. If you're still in doubt about the relevance of Union χ, one of the only things we've seen in the Kingdom Hearts IV trailer is a character first introduced in Union χ...