The trump coup is a contract bridge coup used when the hand on lead (typically the dummy) has no trumps remaining, while the next hand in rotation has only trumps, including a high one that would have been onside for a direct finesse if a trump could have been led. The play involves forcing that hand to ruff, only to be overruffed. A similar motive is met in coup en passant, where indirect finesse is used instead of direct.
In the end position below, spades are trump. The king of spades is onside, but declarer (South) cannot finesse against it because dummy has no trumps remaining.
Declarer takes all three tricks by playing a heart first. When a club is led from dummy, East has nothing but spades remaining and therefore must ruff, and South can overruff with the ace or queen according to which spade East plays.
A trump coup is not possible in a double-finesse position, since declarer with a holding like A-Q-10 over defender's K-J-x would take the first trump trick and then would have to give the defender a free finesse. In effect, a trump coup against a king (or rather, the second-highest remaining trump) must find it guarded by exactly one other trump. (If East held another card instead of the small trump, say a diamond, North-South could still play the hand as just described, but the trump coup would be unnecessary as South could just drop the king of spades instead.)
Similarly, with A-K-J of trumps in hand, a trump coup against Q-x-x on the right is possible, and so on.
To execute a trump coup, declarer must have exactly the same number of trumps as the defender. If declarer had more trumps, entry could not be given to dummy at the critical point when the defender will have only trumps remaining. Sometimes a declarer with too many trumps, but needing to do a trump coup, can set up the desired position by entering dummy and leading a suit he can ruff, to shorten his own trumps.
If the card which is ruffed in order to shorten the trumps would have been a winner, the play is called a grand coup:
|South in 6♣||♠||8|
|♥||6 5 2|
|♦||K 3 2|
|♣||K Q 10 8 5 3|
|♠||10 7 3||
|♠||9 6 4 2|
|♥||J 8 3||♥||Q 10 4|
|♦||J 10 9||♦||A 8 6 5 4|
|♣||J 9 7 4||♣||6|
|Lead:♦J||♠||A K Q J 5|
|♥||A K 7 4|
South, having opened strong 2♣, plays in 6♣. West leads a diamond to East's ace, who returns a heart to South's ace. South plays A-K of trumps and discovers the bad break.
Now, South has to ruff his high spades in dummy twice to shorten its trumps to the same length as West; he cashes two high spades, discarding a heart from dummy, and ruffs the spade queen. Next, he enters his hand with the ♦Q and ruffs another spade. He cashes the ♦K, and reenters his hand with the ♥K. At that point, North and West have only two trumps each - K 10 and J 9 respectively. Either a heart or a spade from South's hand completes the coup.
Often, for a trump coup to work, the key defender must have a suitable distribution in other suits, so that he cannot ruff a declarer's winner prematurely.