Players of Monster Hunter Tri take on the role of a hunter from the hunting Guild, assigned to help Moga Village, a small fishing community that is under threat of monsters from a nearby deserted island. The player does this by completing free hunts on the island, where they collect materials and slay or capture monsters which are converted into resources that can be used to improve the village, and by completing time-limited quests for the Guild, typically slaying or capturing a monster in a specific region, for in-game money (zenny) and material resources that can be used to create equipment that allows them to take on more difficult monsters. Improving the village gives the player access to a farming area and a fishing fleet, from which herbs, mushrooms, bugs and honey can be harvested, companions that can help while on the field, and access to a trader that can provide rare items to the player.
The player's character has 1 base defence and can increase it by the armor created by the player. In particular, the player selects from one of ten weapon classes, primarily divided into melee weapons like swords and hammers, and ranged weapons like bows and bowguns. Each weapon type has a unique style of play when used in the field; sword users need to build up consecutive attacks to increase a combo meter to inflict larger damage on their foes but must keep an eye on their blade's sharpness, while bowmen can perform weak but fast ranged attacks including applying special coatings to their arrows to inflict debuffs on enemies. The player can also equip different sets of armor, which impart attack and defensive bonuses. Some weapons and armor include slots that gem decorations can be slotted into which boosts the item's statistics. Various armor pieces give special skill boosts, and if the player can equip a set that boosts a skill past set thresholds, they can gain additional passive bonuses such as full resistance to fire or poison attacks. The player can swap weapons and armor back in the village.
To progress in the game, the player is typically required to collect parts of the larger monsters that they are assigned to kill, and using those to forge or upgrade weapons and armor. By being able to take on higher-ranked Guild quests with improved equipment, the player will gain access to a larger array of monster parts and world resources that lead to better equipment they can then craft.
In the field, the player has a health and stamina meter (these max to 150 each), as well as an item pouch with limited space where resources like healing potions, bombs, and materials picked up in the field are stored. Certain materials can be combined to make more potent items, such as stronger healing potions. However, once the item pouch is full, the player must choose to discard or swap out items with new ones. As the player hunts, they will take damage from monsters; if their health drops to zero, they are rescued to a base camp where they can set off again, but after falling for a third time, they are returned to the village without any gains made while they were adventuring. The stamina meter drops after performing most intensive actions like running, dodging, or attacking, but restores when the player otherwise is walking around. When the player's stamina is depleted, the character will be forced to stop for a brief period to catch their breath before they can move, leaving them vulnerable to attack. In addition, the maximum stamina that the player has will drop over time while in the field, though certain items can restore the maximum stamina level. While on free hunts, the player can leave the field and return to the village at any time, keeping any items they have collected; on Quest hunts, failure to complete the mission in time or abandoning the quest foregoes any reward items.