The Balmaclellan-born star, who plays Jamie Fraser in the hit time travel show, said he's not even able to tell co-star Caitriona Balfe.
Speaking to Esquire magazine, the 42-year-old said: “Diana Gabaldon [the writer-creator] actually revealed to me how the whole thing’s going to end.
“She emailed me the last few pages of what will be the last book very early on – I think in the first few weeks of shooting and no one else has seen that I think, apart from one other exec producer.
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“Even Caitriona’s not seen it and I’m sworn to secrecy.”
Gabaldon has said in the past there will be ten books total in the Outlander series – and she's already started writing number ten.
The bestselling American author also revealed the series will wrap up in Scotland, around the year 1800, and has promised fans it will be tearjerker, albeit with a happy ending.
She said: “The last book will have a happy ending, though I confidently expect it to leave the readers in floods of tears, anyway.”
In an interview with Square Mile, a men’s luxury lifestyle magazine for London, he was asked if he could think of a moment or experience that influenced who he is as a person today.
He replied: “I guess when I moved to Edinburgh I was living in the countryside as a young child, there were four people in my class at school. And then I moved to Edinburgh, which to me felt like a metropolis. I started going to the theatre.
“Just being in the city was really inspiring. It opened up the world to me, as a teenager. There’s a whole world out there that I want to explore.”
It’s not the first time Heughan has spoken of his love for Scotland’s capital city.
In the past, Heughan said he owes a lot of his success to the Edinburgh Steiner School, where he studied over 20 years ago.
During an interview with the prestigious school a couple of years back, Heughan opened up about his time there and how it helped him as an actor.
He said: ”Looking back on it, It was a really good time, a really warm time, it felt like a really safe environment. There was a real bond between not just the teachers but the students too.”
Heughan also recently admitted he would love to be the next James Bond, saying it’s every actor's dream to play 007.
Speaking to the Radio Times, Heughan recalled auditioning to be the iconic spy prior to Daniel Craig's official casting in October 2005.
“I met [producer] Barbara Broccoli and [Casino Royale director] Martin Campbell,” he said.
“I did a scene and they had the gun from The Man with the Golden Gun on the table and I remember thinking, ‘Wow, this is incredible’.
“It was surreal,” he added. “He’s a character we’ve all grown up with.”