The backpack is a player's personal inventory, allowing for storage for all in-game items. Items are kept in individual slots and can be viewed, moved or deleted from within the backpack interface. Additionally, most tool items can only be used from within the backpack. Items are displayed as pages, with each page displaying 50 items. The backpack can also be sorted by class, quality, loadout slot, and type.
Before the backpack, there was a simple loadout screen for each class (introduced with the Gold Rush Update) which contained only weapon slots. Those classes who had updates released could use it to select between the stock and any achievement items that had been unlocked. The backpack (along with the class loadout screen proper) was introduced into the game some time between the Scout and Sniper vs. Spy Updates, with an initial storage capability of 50 items, even though it could only store the 12 unlocked achievement items at that point (which could be deleted and never re-earned: April 20, 2009 Patch). The Sniper vs. Spy Update introduced the item drop system, which used this extra space. This was originally designed to supplant achievement weapon unlocking entirely, although achievement unlocks were reintroduced a week later due to public outcry. The storage size has since been increased on multiple occasions, and as of the Australian Christmas update in December 2010, the backpack now consists of 300 slots (or 6 pages). The Australian Christmas update also introduced the Backpack Expander tool, enabling the addition of 100 more slots per use to a total of 2000 backpack slots. The Jungle Inferno Update increased the maximum backpack slots from 2000 to 2500, followed by an expansion to 3000 total slots for Smissmas 2017. Free-to-Play Team Fortress 2 players are limited to a single backpack page (50 slots); once they become Premium players with any in-game purchase or the use of an Upgrade to Premium Gift, their backpack is upgraded to the standard capacity.
Items, when moved within the backpack, make a distinct sound based on the properties of the item itself; a distinct sound is usually heard when an item is picked and another when it is put down again. For example, the Eyelander, when moved, produces the sound of a sword being drawn, and Bonk! Atomic Punch makes the sound of a pop top opening when it is picked up and the sound of a soft drink can falling to the ground when it is put down. Some items have unique sound effects, such as Ze Goggles, which make a sound that resembles the goggles turning on and off. Noise Makers play their sound effects.
Clicking on an item displays a context menu, detailing actions that can be done with the item. These include opening the loadout of a class that can equip the item, giving the item a custom name, giving the item a custom description, painting the item, restoring a customized item back to its original state, using the item from inside the backpack, delivering a gift, purchasing a key from the Mann Co. Store, simultaneously purchasing the key and opening a crate, trading the item to another player, selling the item on the Steam Community Market and deleting the item. Hovering over the Go to Loadout option displays a panel showing the classes that can equip the item, while hovering over the Paint option displays a panel showing the available colors and their paint splotches.
- Occasionally, due to various reasons, items may become "invalidly placed" when placing an item on top of a different item so that one of the items disappear (Note: The item is not gone from the backpack. The player must then move around items to make it reappear).
- With "Show quality colors" toggled on, dragging an item into invalid space (off the border, into the header) resets it without its quality color. The quality color returns when switching the page or highlighting the item.
- When "Show quality colors" is enabled, the name of the item in the icon cannot be seen in the equip menu or crafting menu. However, this does not matter as a box appears with the name along with details of the item.
- When both "Show quality colors" and "Show Stock items" are toggled on, the Normal quality stock items are given yellow highlighted borders, as if they are Unique quality.
- It is possible to receive items via Steam trade (not in-game) even if the backpack is full. New items are not shown in the game's backpack and cannot be used in crafting, but can be used in game and can be viewed in the Steam inventory. Removing or trading enough items and resorting to the backpack makes new items fully available. Note that new items can occasionally be removed from the backpack with no warning after entering a game.
- When dragging an item to another page, the pop-up displays the wrong item in relation to where the cursor is.
- Changing the anti-alias settings or resolution from the game's options makes the text when mousing over an item translucent. This is fixed by rebooting the game.
- Upon selecting an item with settable styles and moving the cursor around using the arrow keys, one is able to view the model of applicable items selected, even if their style cannot be set.
- If the 'OK' button is pressed when viewing a item that does not have multiple styles, the game crashes.
- When viewing the context menu of a paintable item, the Paint submenu lists "Paint Can" as a possible color.
- Occasionally, certain item icons may disappear when the player has been in a Casual Mode match previously.
- This can be fixed by using the Sort Backpack dropdown selection.
- When sorted by class, the class order uses the internal order (which is the same as Team Fortress Classic) rather than the more familiar role-based ordering.