Several Attack cards reduce the handsize of opponents by making them discard cards from their hand or return them to the deck—forcing them to start their next turn with fewer than the ordinary 5 cards, and thus reducing the amount they can accomplish on that turn.
Such Attacks are often well defended against by cards that redraw your hand up to a specified size (e.g. Library, Watchtower, Jack of all Trades, Minion, Guide), and by cards that are more likely to achieve their best effects if played in a small hand (Menagerie, occasionally Shanty Town). Horse Traders is also a very effective counter to most handsize attacks, since it increases your handsize after you're attacked. On the other hand, decks built on other cards that reduce their owners' handsize when played, such as Warehouse or Hamlet, are especially vulnerable to these Attacks.
A powerful combo involves playing a handsize attack and then Masquerade—forcing the victims to either discard their bad cards to the attack and pass a good card to the Masquerade, or to potentially discard good cards in order to keep a bad card in hand to pass.
 List of handsize attacks
 Discard to target handsize
These attacks reduce the victim's handsize to a specific number of cards—usually 3, occasionally 4—regardless of how many they had to begin with. This means that additional plays of the Attack after the first don't hurt opponents; they are already down to the target handsize. These can be especially useful to counteract the effects of cards that increase opponents' handsize, like Council Room and Governor, since a single play of the Attack will reduce the victims' handsize almost as effectively as if the handsize increaser had never been played.
- Dominion: Militia
- Intrigue: Minion
- Seaside: Ghost Ship (opponents topdeck cards instead of discarding)
- Prosperity: Goons
- Cornucopia: Followers
- Hinterlands: Margrave
- Dark Ages: Sir Michael, Urchin and Mercenary
- Empires: Catapult, Legionary
- Nocturne: Fear, Poverty
 Stackable handsize attacks
These attacks can be repeated and continue to reduce players' handsize after the first play. These attacks either target specific cards, and will not continue reducing handsize if a player has none of the targeted cards in hand, or they can be prevented from reducing handsize if a player chooses to gain a Curse, or both.
- Dominion: Bureaucrat targets Victory cards
- Intrigue: Torturer allows a player to gain a Curse instead of discarding
- Seaside: Cutpurse targets Copper
- Adventures: Soldier causes opponents to discard 1 card, but not if they have fewer than 4 cards in hand.
- Nocturne: Haunting causes opponents to top-deck 1 card, but not if they have fewer than 4 cards in hand.
- Renaissance: Villain causes opponents to discard 1 card costing or more, but not if they have fewer than 5 cards in hand.
 Attacker's choice
These cards target a specific card (or cards) of the attacker's choice to discard, but do not affect players with fewer than 5 cards in hand:
- Mountebank is not generally considered a handsize attack; it can cause a player to discard a Curse, but this is regarded as a defense against Mountebank's cursing attack rather than a discard attack per se.
- Haunted Woods, from Adventures, forces a player to top-deck their hand when they buy a card; in most situations this has no effect on their current turn, making the card not very effective as a handsize attack, but it can matter when a player wants to retain cards in hand while buying, such as Watchtower, or Night cards from Nocturne.
- Relic and Raid (which, though not technically an Attack, behaves like one) act sort of as a delayed handsize attack; rather than making opponents discard, they prevent them from drawing the next time they try to.
- Haunted Castle, from Empires, has an on-gain effect that acts like a handsize attack, but the card is not actually an Attack card and the effect under normal circumstances can only be used once.