In which Shakespeare play does the character Marcellus say, "Something is rotten in the state of Denmark"?

The answer is Hamlet. In Shakespeare's play Hamlet, the character Marcellus says the famous line "Something is rotten in the state of Denmark." This line is said in Act I, Scene IV, when Marcellus and Horatio are keeping watch on the battlements of Elsinore Castle and witness the appearance of the ghost of King Hamlet. Marcellus's remark reflects the unrest and corruption that has plagued Denmark under King Claudius's rule, setting the stage for the central themes of deceit, revenge, and moral decay that unfold throughout the play.