What is the counter-curse to Sectumsempra?

Question

I was wondering if anyone knew the counter-curse to Sectumsempra. Just in case after I teach my enemy a lesson, I might want to not kill them.

Answer 1

We don't know exactly what the countercurse is, only that there is one.

The main thing we know about it in canon (see the HP Lexicon page on the so-called "Sectumsempra countercurse") is from the scene in book 6, chapter 24, where Snape uses it to heal Malfoy. As luck would have it, this passage of the book is available on Pottermore:

The door banged open behind Harry and he looked up, terrified: Snape had burst into the room, his face livid. Pushing Harry roughly aside, he knelt over Malfoy, drew his wand and traced it over the deep wounds Harry’s curse had made, muttering an incantation that sounded almost like song. The flow of blood seemed to ease; Snape wiped the residue from Malfoy’s face and repeated his spell. Now the wounds seemed to be knitting.

Harry was still watching, horrified by what he had done, barely aware that he too was soaked in blood and water. Moaning Myrtle was still sobbing and wailing overhead. When Snape had performed his counter-curse for the third time, he half lifted Malfoy into a standing position.

"You need the hospital wing. There may be a certain amount of scarring, but if you take dittany immediately we might avoid even that … come …"

So about all we know of this spell is that it's an incantation that sounds almost like song and that it apparently needs to be performed three times to be effective.

The healing of George Weasley in book 7 (also mentioned in the above-linked HP Lexicon page) happens off-stage, so we get no further info from this scene:

Glad of something to do, Harry pulled him free, then headed through the empty kitchen and back into the sitting room, where Mrs. Weasley had staunched his bleeding now, and by the lamplight Harry saw a clean, gaping hole where George’s ear had been.

"How is he?"

Mrs. Weasley looked around and said, "I can’t make it grow back, not when it’s been removed by Dark Magic. But it could have been so much worse... He’s alive."

Answer 2

There is one, but Snape will probably have to teach it to you.

As we know Harry learned the spell from potions book that had once belonged to Snape, and Snape was the "inventor" of the spell. It stands to reason that Snape was then also the inventor of the counter-curse.

The scene from the film comes from the description in the books

The door banged open behind Harry and he looked up, terrified: Snape had burst into the room, his face livid. Pushing Harry roughly aside, he knelt over Malfoy, drew his wand and traced it over the deep wounds Harry’s curse had made, muttering an incantation that sounded almost like song. The flow of blood seemed to ease; Snape wiped the residue from Malfoy’s face and repeated his spell. Now the wounds seemed to be knitting.

Harry was still watching, horrified by what he had done, barely aware that he too was soaked in blood and water. Moaning Myrtle was still sobbing and wailing overhead. When Snape had performed his counter-curse for the third time, he half lifted Malfoy into a standing position.

_{Harry Potter and the Half-Blood Prince}