Pier Gerlofs Donia (c.1480 – 1520) was a Frisian warrior, pirate, and rebel. He is best known by his West Frisian nickname "Grutte Pier" ("Big Pier" in the Old Frisian spelling), or by the Dutch translations "Grote Pier" and "Lange Pier", or, in Latin, "Pierius Magnus", which referred to his legendary size and strength.

His life is mostly shrouded in legend. Based upon a description now attributed to Pier’s contemporary Petrus Thaborita, the 19th-century historian Conrad Busken Huet wrote that Grutte Pier was

Grutte Pier was born Pier Gerlofs Donia (Pier Gerlofs), around 1480 in Kimswerd near the city of Harlingen, Wonseradeel in Friesland, Netherlands. Pier Gerlofs was one of at least four children born to Fokel Sybrants Bonga and Gerlof Piers. Pier's mother Fokel was the daughter of the Schieringer noblemen Sybrant Doytsesz. Bonga of Bongastate, Kimswerd.

Pier married Rintsje Syrtsema and they had two children, a son named Gerlof and a daughter named Wobbel, who were born around 1510. Pier died in 1520, and in 1525, Pier's mother appointed in her will Pier's brother, Sybren, as guardian of Pier's children, who still were minors. Pier and Pier's brother-in-law Ane Pijbes (husband of Tijdt Gerlofs), were partners in the farming estate of Meyllemastate in Kimswerd.

Pier was the direct descendant of the Frisian chieftain Haring Harinxma (1323–1404), a Schieringer and potestate of Westergo, and the third cousin of Jancko Douwama.

Wijerd Jelckama often is described by 18th and 19th century authors as the nephew of Grutte Pier. Contemporary Worp van Thabor identifies him simply as Wierd van Bolsward. Modern authors such as J.J. Kalma doubt the nephew connection and Brouwer in the Encyclopedia of Friesland states that Wierd was not Grutte Pier's nephew but instead his "lieutenant".

Approximately 7 kilometres (4.3 mi) to the north-east of Donia's village of Kimswerd, in the city of Franeker, the Black Band, a Landsknecht regiment in the service of George, Duke of Saxony was quartered. The regiment was charged with suppressing the civil war between the Vetkopers, who opposed Burgundian and subsequently Habsburg rule, and the Schieringers. The Black Band were notorious as a violent military force; when their pay was insufficient or lacking, they would extract payments from local villagers and on 29 January 1515, the Black Band plundered Donia's village, then allegedly raped and killed his wife, Rintze Syrtsema, and burnt to the ground both the village church and Donia's estate. Seeking revenge, Pier started a guerrilla war campaign against the Habsburgs and allied himself with Charles of Egmond, Duke of Guelders (1492–1538), the foremost opponent of the Habsburgs.

Pier's armed band, known as the Arumer Zwarte Hoop (English: Arumer Black Heap, as in "group/mass of"), were pirates mainly active against the Hollanders and Burgundians at sea. He managed to capture many English and Dutch ships, mainly on the Zuider Zee (today's ‘IJsselmeer’). In the biggest battle of his career, in 1515, he captured 28 Dutch ships, which earned him the nickname "Cross of the Dutchmen".[citation needed] Erasmus criticized Grutte Pier's exploits.

Pier targeted ships that travelled the Zuider Zee and was very active in 1517, when he used his "signal ships" to attack ships in the region of the West Frisian coast, to which he also transported Geldrian forces, setting them ashore at Medemblik. Pier bore a personal enmity to Medemblik and its inhabitants as, in earlier years, soldiers from Medemblik had cooperated with the Holland army commanded by Duke Charles, the future Emperor. In March 1498, Medemblik was where representatives of the Schieringers met the Saxon ruler duke Albrecht to request Saxon protection from the Vetkopers — a request that resulted in the Saxon occupation of Friesland, Netherlands. On 24 June 1517, Grutte Pier and his Arumer Zwarte Hoop, consisting of some 4,000 soldiers from Frisia and Guelders, sailed to West Frisia, passing Enkhuizen, landing near Wervershoof and advancing to Medemblik. They swiftly captured Medemblik, killing many inhabitants and taking many others prisoner. Some were released on payment of a high ransom. Some of the town's inhabitants fled and found safety at Medemblik Castle. The castle's governor, Joost van Buren, succeeded in keeping the aggressors outside the castle walls. Unable to take the castle, the Arumer Zwarte Hoop plundered the town and set it on fire. With most houses made of wood, the town, including the church, monastery and town hall, were completely razed. After this partial victory, Pier and his army stormed both Nieuwburg and Middleburg Castle near Alkmaar, plundered and set them on fire, leaving only ruins.

In 1517, the Arumer Zwarte Hoop captured the town of Asperen, slaughtering virtually all its inhabitants. They then used the heavily fortified city as a base until driven out by Holland's Stadhouder. In response to the attacks on Medemblik and Alkmaar and the failure of the Captain General of Amstelland, Waterland and Gooiland to defend his territories, the Stadhouder of Holland agreed to fit out a war fleet in July 1517. The fleet came under the supreme command of Anthonius van den Houte, Lord of Vleteren, appropriately titled "Admiral of the Zuider Zee". In the name of Charles V, van den Houte announced he would free the region of Frisian and Gelder piracy. Although van den Houte was initially successful, with some of the Frisian vessels being burnt near Bunschoten, Grutte Pier responded by seizing 11 of Holland's ships in a battle off the coast near Hoorn in 1518.