Captal de Buch (later Buché) was an archaic feudal title in Gascony, captal from Latin capitalis "prime, chief" in the formula capitales domini or "principal lords." Buch was a strategically located town and port on the Atlantic, in the bay of Arcachon. As an actual title the word "captal" was used only by the seigneurs of Trene, Puychagut, Épernon and Buch.
When Pierre, the seigneur of Grailly (ca 1285 – 1356) married Asalide (the captaline de Buch), the heiress of Pierre-Amanieu de Bordeaux, captal de Buch, in 1307, the title passed into the Grailly family, a line of fighting seigneurs with origins in Savoy.
The most famous of the Captals de Buch was Pierre's grandson, Jean III de Grailly, captal de Buch (1343–1377), a cousin of the Count of Foix who was a military leader in the Hundred Years' War, praised by the chronicler Jean Froissart as an ideal of chivalry.