Luigi's Diary is an item in Paper Mario. It is found in a basement of the Mario Bros.' House. This diary of Luigi's can be accessed after Mario gets the Ground Pound ability in Chapter 3. Mario can enter the basement from the brothers' room by ground pounding the floor. After every chapter, there will be a new entry in the diary that can be quite humorous to read. Before Chapter 3, Mario can't access Luigi's Diary, but he can tell that Luigi is writing in it, because there are music notes coming out through the floor of the brothers' room. However, during Chapter 3, if Mario returns to Toad Town from Boo's Mansion after obtaining the Ground Pound and decides to enter the basement at this point before having finished Chapter 3, Luigi will still be in there and become stunned by Mario's presence for a brief moment. He will flee and take his diary with him. Even if this happens, if Mario enters the basement after leaving the grounds of his house and returning later, he can still find the diary.
Luigi's Diary also makes small references to spinoff games from the Mario franchise, which includes the Mario Golf, Mario Tennis, and Mario Party series, and even a note of Luigi possibly getting his own game, which later on became Luigi's Mansion.