UNDERTALEプラスはデベロッパーからアドオンを集めています。最初のバージョンでは4つのアドオンが使え、だんだんアドオンが増えていきます。
[Release:]
ゲーム全体のバトルをDELTARUNE化する。
作LuigiUTMC
[Every turn, your SOUL changes color!]
作LuigiUTMC

[Feeling like the master of the Burnt Pan? Test your quad-hitting skill!]
作LuigiUTMC

[Embark on a 6-hour long journey across the desert. One misstep, and it's back to the start with you!]
作CiblesGDとChrono Sariel

[Post-Release:]

[Play Undertale online with your friends! Features custom skins and UI!]
作LandimizerとLuigiUTMC

[Ever wanted Undertale to be controlled by your Twitch Chat? Embrace that dumb fantasy with Chaos addon!]
作LuigiUTMC

[Could you give me a second? I'm fighting Undyne the Undying in the Ruins rock puzzle room...]
作LuigiUTMC

[Venture into a more menacing underground... Who knows what's in store for you there?]
作[The Undertale Plus Development Team]

???