A musketeer (French: mousquetaire [muskətɛʁ] ^ⓘ) was a type of soldier equipped with a musket. Musketeers were an important part of early modern warfare from the mid 15th to mid 19th centuries, particularly in Europe, where they normally comprised the majority of their infantry after 1600. The musketeer was a precursor to the rifleman. Muskets were replaced by breech loading rifles as the almost universal firearm for modern armies during the period 1850 to 1870. The traditional designation of "musketeer" for an infantry private survived in the Imperial German Army until World War I.

The hand cannon was invented in Song dynasty China in the 12th century and was in widespread use there in the 13th century. It spread westward across Asia during the 14th century. The hand cannon evolved into the arquebus that appeared in Europe and the Ottoman Empire during the 15th century. The term musket was originally used to describe a heavy arquebus capable of penetrating heavy armor. Although this heavy version of the musket fell out of use after the mid-16th century with the decline of heavy armor, the term "musket" remained as a general descriptor and lent its name to the infantry type known as the musketeer.

Musketeers are primarily associated with infantry in early modern Europe, and were employed by many European nations.

In the Spanish army, the tercio or the Spanish square was a mixed infantry formation that theoretically could number up to 3,000 pikemen, swordsmen and musketeers; although it was usually much smaller on the battlefield. It was effective in its era, capitalizing on the close-quarter impact of the pike combined with the long-range projectile capabilities of the musket. It resembled a loosely formed phalanx in function, but was far more flexible and deadly. Musketeers were developed by the Spanish during the Italian Wars so as to deal with the heavily armored French Gendarmes. An arquebus was not powerful enough to take down an armored knight, but a wall gun was. Spanish field commanders wanted to bring the firepower of a small wall gun onto the battlefield yet have it be as maneuverable as an arquebus. The solution was a bigger arquebus, but the additional weight made it extremely difficult to support the barrel during aiming and firing; hence, the musket rest, the precursor to the monopod. Furthermore, musketeers were the first infantry to give up armor entirely. Other than the musket rest, the musketeer's equipage was upgraded from a powder flask to a bandolier. Due to the difficulty in manipulating the musket rest and the strength needed to handle the heavier gun, musketeers were stronger men and paid more than the rest of the infantry.

The Musketeers of the Guard were a junior unit, initially of roughly company strength, of the military branch of the Royal Household. They were created in 1622 when Louis XIII furnished a company of light cavalry (the "carabiniers", created by Louis' father Henry IV) with muskets. Musketeers fought in battle both on foot as infantry and on horseback as dragoons. At the Battle of Fontenoy in 1745 the King's Musketeers served as regular cavalry, charging British infantry with drawn swords.

As one of the junior units in the Royal Guard, the Musketeers were not closely linked to the royal family. Traditional bodyguard duties were in fact performed by the Garde du Corps and the Cent-suisses. Because of its later establishment, the Musketeers were open to the lower classes of French nobility or younger sons from noble families whose oldest sons served in the more prestigious Garde du Corps and Chevau-legers (Light Horse). The Musketeers, many of them still teenagers, soon gained a reputation for fighting spirit and unruly behaviour.

Their high esprit de corps gained royal favor for the Musketeers, and they were frequently seen at court and in Paris. Shortly after their creation, Cardinal Richelieu created a bodyguard unit for himself. So as not to offend the king with a perceived sense of self-importance, Richelieu did not name them Garde du Corps like the king's personal guards, but rather Musketeers after the Kings' junior guard cavalry. This was the start of a bitter rivalry between the two corps of Musketeers. At the cardinal's death in 1642, the company passed to his successor Cardinal Mazarin. At Mazarin's death in 1661, the Cardinal's Musketeers passed to Louis XIV, contrary to the wishes of both the King's Musketeers and the Cardinal's Musketeers themselves. The Musketeers were subsequently reorganized as a guard cavalry regiment of two companies. The King's Musketeers became the first company, popularly known as "Grey Musketeers" (mousquetaires gris), while the Cardinal's Musketeers became the second company, known as "Black Musketeers" (mousquetaires noirs) for riding grey and black horses, respectively. From their establishment, the musketeers wore blue cloak-like cassocks, lined with red and edged with silver embroidery. From 1688, the cassocks were replaced by smaller soubrevestes or sleeveless coats in the same colours. In the early decades of the corps, the musketeers had worn civilian dress under their cassocks, according to personal taste and means, but in 1677 a scarlet uniform was adopted.

In terms of recruitment, entry into the Musketeers was much sought after by those sons of the aristocracy who did not possess the quarterings of nobility required for the Garde du Corps and Chevau-legers. These two senior guard units were closed to all but the highest ranking and wealthy noble families. Accordingly for lesser gentry, or ambitious commoners, service in the Musketeers was the only way to join a mounted unit in the royal household and perhaps catch the King's eye. However, enlistment did require both letters of recommendation and evidence that a recruit had the family means to support the costs of service. These included the provision of horses, swords, clothing, a servant and equipment. Only the musket, the sleeveless soubreveste and the distinctive blue cassock were provided by the monarch.