A particular form of ViralMarketing.

You have a science fiction movie coming out. What can do you do to promote it that goes that extra mile?

You do InUniverseMarketing. You post "found" documents from within the universe. A [[Film/{{Watchmen}} "NBS Nightly News" broadcast from 1972 in your world]]? Great. [[Film/{{Idiocracy}} Advertising a drink from your setting]]? Yup. [[LightNovel/HaruhiSuzumiya Showing a half hour of dull security camera footage with maybe thirty seconds of profoundly disturbing stuff going on]]? Perfect. [[Series/TwinPeaks The diary of one of the characters, with pages torn out]]? [[{{Crunchtastic}} Synergylicious!]]

Distinct from the AlternateRealityGame in that there is no 'game', per se. Rather, all there is is the document.
Websites that use InUniverseMarketing are usually LoggingOntoTheFourthWall.

Not all ViralMarketing qualifies as InUniverseMarketing; the product being advertised must be fictional, and the "found" document must be set in said universe.

Examples should go under ViralMarketing; this entry just defines the term.