Paranormal Activity is a 2007 American supernatural horror film produced, written, directed, photographed, and edited by Oren Peli. It centers on a young couple (Katie Featherston and Micah Sloat) who are haunted by a supernatural presence inside their home. They then set up a camera to document what is haunting them. The film uses found-footage conventions that were mirrored in the later films of the series.

Originally developed as an independent feature and given film festival screenings in 2007, the film was shot for $15,000. It was then acquired by Paramount Pictures and modified, particularly with a new ending that cost an additional $200,000. It was given a limited U.S. release on September 25, 2009, and then a nationwide release on October 16, 2009. The film earned nearly $108 million at the U.S. box office and a further $85 million internationally for a worldwide total of $194 million. Paramount/DreamWorks acquired the U.S. rights for $350,000. It is often cited as the most profitable film ever made, based on proportionate return on investment, although such figures are difficult to verify independently as this is likely to exclude marketing costs.

The film is the first entry in the Paranormal Activity film series. A parallel sequel, Paranormal Activity 2, was released in 2010.