The Countess of Pembroke's Arcadia, also known simply as the Arcadia, is a long prose pastoral romance by Sir Philip Sidney written towards the end of the 16th century. Having finished one version of his text, Sidney later significantly expanded and revised his work. Scholars today often refer to these two major versions as the Old Arcadia and the New Arcadia. The Arcadia is Sidney's most ambitious literary work by far, and as significant in its own way as his sonnets.

Sidney's Arcadia has a history that is unusually complex even for its time.

Sidney may have begun an early draft in the late 1570s, when he was in his twenties. His own comments indicate that his purpose was humble; he asserts that he intended only to entertain his sister, Mary Herbert (Countess of Pembroke from 1577). This version is narrated in chronological order, with sets of poems separating the books from each other. It seems likely that Sidney finished this version while staying at Herbert's estate during a temporary eclipse at court in 1580.

In 1588, two years after Sidney's death, Fulke Greville appears to have appealed to Francis Walsingham to prevent an unauthorized publication of parts of the original, as we learn from a letter that also serves as evidence for the circulation of Arcadia in manuscript form:

Sir this day one Ponsonby a bookbinder in Paul's Churchyard, came to me, and told me that there was one in hand to print, Sir Philip Sidney's old Arcadia asking me if it were done with your honour's consent or any other of his friends/ I told him to my knowledge no, then he advised me to give warning of it, either to the Archbishop or Doctor Cosen, who have as he says a copy of it to peruse to that end/ Sir I am loath to renew his memory unto you, but yet in this I must presume, for I have sent my Lady your daughter at her request, a correction of that old one done 4 or 5 years since which he left in trust with me whereof there is no more copies, and fitter to be printed than that first which is so common, notwithstanding even that to be amended by a direction set down under his own hand how and why, so as in many respects especially the care of printing it is to be done with more deliberation,

Sidney's original version was all but forgotten until 1907, when the antiquarian Bertram Dobell discovered that a manuscript of the Arcadia he had purchased differed from published editions. Dobell subsequently acquired two other manuscripts of the old Arcadia: one from the library of the Earl of Ashburnham and one that had belonged to Sir Thomas Phillipps. This version of the Arcadia was first published in 1926, in Albert Feuillerat's edition of Sidney's collected works.

The version of the Arcadia known to the Renaissance and later periods is substantially longer than the Old Arcadia. In the 1580s, Sidney took the frame of the original story, reorganized it, and added episodes, most significantly the story of the just rebel Amphialus. The additions more than double the original story; however, Sidney had not finished the revision at the time of his death in 1586.

The New Arcadia is a romance that combines pastoral elements with a mood derived from the Hellenistic model of Heliodorus. A highly idealized version of the shepherd's life adjoins, on the other hand and not always naturally (in its literary sense), stories of jousts, political treachery, kidnappings, battles, and rapes. As published, the narrative follows the Greek model: stories are nested within each other, and different storylines are intertwined.