Along with a series of glamorous images she posted to Instagram, Kim wrote that her and her reported 30 guests, who have spent several days partying on the island, "danced, rode bikes, swam near whales, kayaked, watched a movie on the beach and so much more".
It is understood the island was in Tahiti.
A source told the New York Post: "The group was limited to Kim's closest family and friends. Everyone on the trip was repeatedly tested for COVID before they left in the middle of the night on a private Boeing 777.
"Once on the island they had lavish dinner after dinner, spa treatments and hikes through the nature sanctuary. There was no expense spared. Everyone was banned from posting on social media by Kim to ensure her privacy."
Kim also revealed she asked her guests to undergo quarantine before they all congregated for the luxurious holiday.
"After 2 weeks of multiple health screens and asking everyone to quarantine, I surprised my closest inner circle with a trip to a private island where we could pretend things were normal just for a brief moment in time," she said on Instagram.
"I realize (sic) that for most people, this is something that is so far out of reach right now, so in moments like these, I am humbly reminded of how privileged my life is."
Kim's sisters Kourtney, Khloe, half-sister Kendall Jenner and mother Kris Jenner were present in the pictures, as well as Tristan Thompson, Corey Gamble and Scott Disick.
Interestingly, it is not known if Kanye West or Kylie Jenner attended the fanfare given they weren't seen in any pictures or didn't post from the island on their own accounts.
Members of the family posted several relaxed pics from their exotic trip away, with Kendall posing as she sipped on a cocktail while Kim, Khloe and Kourtney looked relaxed as they frolicked on a beach in their bikinis.
Kim also posted a host of breathtaking bikini pictures solo as she soaked up the sunshine, captioning them, "ThisIs40".
The celebrations will no doubt appear on the upcoming – and final – season of Keeping Up With The Kardashians, which streams on Binge.
Ahead of the secret trip, Kim's family hosted a surprise party for the KKW Beauty founder where they re-created moments from her birthdays over the years.
Kim took to Instagram to tell her fans what happened inside the bash.
"They re-created all of my special birthdays throughout my life from looking at all of our old home videos," Kim wrote on Instagram after her birthday bash on October 21.
"They re-created the party, The same balloons, the same cake, everything! My dad video taped and documented our whole lives so they showed each of the videos of my birthdays on a big screen behind the party."
"I entered into the room that started off at my first birthday party with the same ponies and the same cake, and my dad wishing me a happy 1st birthday."
"Then as I entered in the room I ended up at my second birthday then my fourth birthday leading up to my 10th birthday where my sisters did the same exact dance that they did for me when I turned 10."
