Peter I (French: Pierre; c. 1187 – 26 May 1250), also known as Peter Mauclerc and Peter of Dreux, reigned as Duke of Brittany alongside his wife Alix from 1213 to 1221, and was regent of the duchy for his minor son John I from 1221 to 1237. As duke he was also 1st Earl of Richmond from 1218 to 1235.

Peter was the second son of Robert II, Count of Dreux and Yolande de Coucy. The former was in turn the son of Robert I, Count of Dreux, a younger brother of Louis VII of France. Peter was thus a Capetian, a second cousin of Louis VIII of France.

Despite being of royal descent, as the younger son of a cadet branch Peter's early prospects were that of a minor noble, with a few scattered fiefs in the Île-de-France and Champagne. He was initially destined for a career in the clergy, which he later renounced, earning him the nickname Mauclerc (French: mauvais clerc, bad-cleric). He broke the convention of ecclesiastical heraldry by placing on the canton of his paternal arms the ermine, then reserved for the clergy.^{[citation needed]}

In 1212 King Philip II of France needed to find a weak and faithful ruler for Brittany. The duchy lay athwart the sea lanes between England and the English territories in Gascony. Furthermore, it bordered on Anjou and Normandy, which the English had lost ten or twelve years before and were eager to recover. It was being ruled with less than a strong hand by Guy of Thouars, as regent for his young daughter Alix. Also worrisome was that Alix's older half-sister Eleanor, Fair Maid of Brittany, was in an English prison.

King Philip thus broke off the betrothal of Alix and the Breton lord Henry of Penthièvre, and turned to his French cousin Peter, then in his early twenties. Peter married Alix, and on 27 January 1213, paid homage to the king for Brittany.

There is some ambiguity regarding whether Peter should be considered duke or count. The duchy was legally held by his wife. The king of France and the Pope (and their courts) always addressed him as count, but Peter in his own charters called himself duke.

In 1214 King John of England had assembled a formidable coalition against the French. He landed in Poitou while Otto IV of Germany prepared to invade from the north. John chased off some French forces in the north of Poitou, and then moved to the southern edge of Brittany, opposite Nantes. Peter drove him off after a brief skirmish but did nothing to hinder John's subsequent movement up the Loire valley where he took a few Breton fortresses and then besieged La Roche-aux-Moines. John's Poitevin vassals, however, refused to fight against a French force led by Prince Louis of France. Meanwhile, Otto's army was crushed at Bouvines, and the entire invasion foundered.

It is not clear why John attempted to capture Nantes, even less why he would do so the hardest way, via the very well-defended bridge across the Loire. Nor is it clear why Peter declined to harass his forces from the rear as John marched east. A likely explanation is that the two had come to some sort of agreement whereby John would leave Brittany alone for the moment, and in return the Bretons would not hinder him elsewhere.