All of the early editions are anonymous. The first indication that the author of the play was Kyd was in 1773 when Thomas Hawkins, the editor of a three-volume play-collection, cited a brief quotation from The Spanish Tragedy in Thomas Heywood's Apology for Actors (1612), which Heywood attributes to "M. Kid". The style of The Spanish Tragedy is considered such a good match with Kyd's style in his other extant play, Cornelia (1593), that scholars and critics have universally recognised Kyd's authorship.
In 2013, scholar Douglas Bruster theorised that some awkward wordings in the "Additional Passages" of the 1602 fourth edition resulted from printers' errors in setting type from the (now lost) original manuscript. Furthermore, after examining the "Hand D" manuscript (widely accepted as in Shakespeare's handwriting) from the play Sir Thomas More, Bruster opined that the speculated printers' errors could have resulted from reading a manuscript written by someone with Shakespeare's "messy" handwriting, thus bolstering the likelihood that Shakespeare wrote the Additional Passages.