Official ruleBase game, 6th edition (2025) and 5th edition
Can you trade with the bank on another player's turn in Catan?
NoShort answer
No. Only the player whose turn it is can trade with the supply or a port. Everyone else may only trade with the active player, not with each other and not with the supply.
The rule
All trading belongs to the active player’s turn.
- Trading with players: the active player can trade with anyone. Other players can trade only with the active player.
- Trading with the supply: 4 of the same resource for 1 of a different resource, or 3:1 or 2:1 at a port where you have a building. Only on your own turn.
Example
During Ben's turn, Ana wants to swap 4 wool for 1 ore with the supply. She has to wait for her own turn. She could, though, offer Ben wool for ore.
Why players argue about it
Supply trades feel like they don't affect anyone else, so many groups allow them any time. Some 5–6 player rules also let a second player trade with the supply during each turn, and people carry that habit over to the base game.