A bad quarto, in Shakespearean scholarship, is a quarto-sized printed edition of one of Shakespeare's plays that is considered to be unauthorised, and is theorised to have been pirated from a theatrical performance without permission by someone in the audience writing it down as it was spoken or, alternatively, written down later from memory by an actor or group of actors in the cast – the latter process has been termed "memorial reconstruction". Since the quarto derives from a performance, hence lacks a direct link to the author's original manuscript, the text would be expected to be "bad", i.e. to contain corruptions, abridgements and paraphrasings.

In contrast, a "good quarto" is considered to be a text that is authorised and which may have been printed from the author's manuscript (or a working draft thereof, known as his foul papers), or from a scribal copy or prompt copy derived from the manuscript or foul papers.

The concept of the bad quarto originates in 1909, attributed to A W Pollard and W W Greg. The theory defines as "bad quartos" the first quarto printings of Romeo and Juliet, Henry V, The Merry Wives of Windsor and Hamlet, and seeks to explain why there are substantial textual differences between those quartos and the 1623 printing of the first folio edition of the plays.

The concept has expanded to include quartos of plays by other Elizabethan authors, including Peele's The Battle of Alcazar, Greene's Orlando Furioso, and the collaborative script, Sir Thomas More.

The theory has been accepted, studied and expanded by many scholars; but some modern scholars are challenging it and those, such as Eric Sams, consider the entire theory to be without foundation. Jonathan Bate states that "late twentieth- and early twenty-first century scholars have begun to question the whole edifice".