Rotten Tomatoes reports a score of 85% based on 13 reviews, with an average rating of 7.61/10. Nathan Lee of "The Village Voice" found the film to be a "nasty number" that was both "repellent" and "uncompromising" while Harry Knowles of Ain't It Cool News called it "off the hook awesome" and "a traumatically entertaining experience." Jason Buchanon of AllMovie gave the film a score of 4/5, and called it "a pitch-perfect balance of grinding tension and inventive gore" while John Fallon of Arrow in the Head gave it a perfect score of 10/10, and deemed it "a real horror film; character driven, suspenseful as hell, smarter than the norm and gory as fuck." Brad Miska of Bloody Disgusting gave "Inside" a top score of 5/5, labeling it "one of the scariest movies I have ever seen in my life" before concluding, "This is quite possibly the most violent, realistic and bloody slasher film ever assembled, ending on such a gut-wrenching note that there's not a single person who could walk out of that theater feeling normal." Bloody Disgusting later ranked the film twelfth in their list of the Top 20 Horror Films of the Decade, with the article saying, "One of the most audacious, brutal, unrelenting horror films ever made, "Inside" is perhaps the crown jewel of the new wave of extreme French horror films." "Inside" was called "the most visceral slasher film of all time" by Steve Barton of Dread Central, who awarded the film a score of 4½ out of a possible 5 and wrote, "The film may be cruel, but it's executed with pure artistry. Haunting images carry their own grotesque beauty and there is real emotional depth anchored by a pair of powerhouse performances from the dueling ladies." "Inside" was deemed an "excellently paced suspense movie" by Kevin Carr of Film School Rejects while Adrian Halen of Horror News praised it as "an amazing film" that "no self-respecting horror fan should go without seeing." Sarah Gopaul of "Popjournalism" found "Inside" to be "a gut-wrenching exhibition of intensity and gore, propelled by strong performances from Dalle and Paradis." Steve Dollar of "The New York Sun" opined that the "darkly stylish and shockingly violent" film was the "essence of grindhouse" and "one of the most edge-of-the-seat, fingers-over-the-eyes flicks in recent memory."