Marble Hornets (abbreviated as MH) is an alternate reality game YouTube web series inspired by the Slender Man online mythos. The first video was posted on YouTube on June 20, 2009, following a post that its creator, Troy Wagner, created on the Something Awful forum the previous day. It was the first web series to be created around the character, with the first episode premiering exactly 10 days after the appearance of the original images of Slender Man on the Something Awful forums, and the first Marble Hornets affiliated post being 9 days after the character's creation.To date, there are 92 videos on the main channel (87 entries). The series also has 38 accompanying videos from a side-channel, "totheark". These videos, as well as the eponymous "totheark", have been featured multiple times throughout the story. As of December 17, 2022, the principal channel has over 113 million views, with over 600K subscribers. The cryptic side channel "totheark" has over 100K subscribers, and 15 million views. It was filmed in Tuscaloosa, Alabama, and other nearby areas.

On August 3, 2015, a follow-up series titled Clear Lakes 44 was uploaded onto the Marble Hornets channel. In April 2016, Clear Lakes 44 was cancelled after the members of the creative team went their separate ways, as confirmed by Wagner. On October 16, 2016, a successor to Clear Lakes 44, titled ECKVA, was launched. On December 27, 2017, Wagner posted a new photo to his Twitter page, teasing a comic related to Marble Hornets slated for a 2018 release. The first issue released on February 25, 2019, with a total of six being slated for the series, with 5 main story comics, and a special "totheark" comic. Five have been released as of 2023, the latest being Issue 4: Can You See Me, published on March 31, 2023. The series is illustrated by Canadian artist Jackie Reynolds.

In 2015, a film spinoff Always Watching: A Marble Hornets Story was released. Critical reception was overwhelmingly negative.