Items

ITEM button visible inside battle.

Items can be bought, picked up, and used throughout Undertale. The protagonist can carry up to 8 items at a time in their inventory, equip 2 items (a weapon and an armor), and store 10 items in each of the two Dimensional Boxes, allowing for a maximum of 30 items to be carried at a time. Items can be sold at the Tem Shop in Temmie Village.

Items usually have regular names which are visible in the inventory, as well as shortened names visible in battle, as a reference to old game translations. Names are often shortened in a comical way, but have a different short name displayed in Serious Mode. Since original Japanese games did not shorten item names, Undertale's Japanese translation does not shorten names in battle.

Consumables

Consumables are items which restore a certain amount of HP or give other boosts to the protagonist's stats like invulnerability or speed.

Equippable items

Players can equip weapons and armor by selecting them from the protagonist's inventory. This allows them to increase certain stats in encounters. Players cannot de-equip the starting weapon and armor the protagonist has in the beginning of the game, they can only change them by equipping another kind of weapon or armor.

Some weapons and armor come in "sets", each appearing to be scattered belongings of one of the seven humans who fell down before the protagonist:

• Toy Knife and Faded Ribbon in the Ruins,
• Tough Glove and Manly Bandanna in Snowdin,
• Ballet Shoes and Old Tutu in Waterfall,
• Torn Notebook and Cloudy Glasses from Gerson,
• Burnt Pan and Stained Apron in Hotland,
• Empty Gun and Cowboy Hat from Bratty and Catty, and
• Worn Dagger and Heart Locket in Asgore's Home.

The battle with Omega Flowey displays items that are thematically similar to these sets during the SOULs' interference (knives, gloves, ballet shoes, notebooks, frying pans, gun).

Weapons

Weapons are equippable items which increase the protagonist's AT stat. They determine how the interface when selecting the FIGHT button in battle looks like, with different weapons having different amount of bars appearing when landing a hit and different attack animations. Some weapons have their attacks appear yellow and make a different noise if the player hits all of the sliding bars in the center when attacking. The protagonist's starting weapon is Stick.

Armor

Armor are equippable items which increase the protagonist's DF stat. Certain armor items have other effects, such as healing every few turns, increased invulnerability, or even higher AT. The protagonist's starting armor is the Bandage, which becomes a consumable item once unequipped.

Miscellaneous

Miscellaneous items are items that cannot be consumed or equipped, but have a variety of other effects. They are often used for plot-related purposes.

There is an additional item named "0" with the description of "If you are reading this, I messed up somehow." It is used to represent empty inventory slots, and is not meant to be normally visible without glitches. Dropping it or viewing it in battle crashes the game.