A grenadier (/ˌɡrɛnəˈdɪər/ GREN-ə-DEER, French: [ɡʁənadje] ^ⓘ; derived from the word grenade) was historically an assault-specialist soldier who threw hand grenades in siege operation battles. The distinct combat function of the grenadier was established in the mid-17th century, when grenadiers were recruited from among the strongest and largest soldiers. By the 18th century, the grenadier dedicated to throwing hand grenades had become a less necessary specialist, yet in battle, the grenadiers were the physically robust soldiers who led vanguard assaults, such as storming fortifications in the course of siege warfare.

Certain countries such as France (Grenadiers à Cheval de la Garde Impériale) and Argentina (Regiment of Mounted Grenadiers) established units of Horse Grenadiers, and for a time the British Army had Horse Grenadier Guards. Like their infantry grenadier counterparts, these horse-mounted soldiers were chosen for their size and strength (heavy cavalry). In modern warfare, a grenadier is a soldier armed with a grenade launcher, either as a standalone weapon or attached to another service weapon.

The concept of troops being equipped with grenades dates back to the military of the Ming dynasty, when Chinese soldiers stationed on the Great Wall used thunder crash bombs. The earliest references to soldiers using grenades in European armies dates back to the early modern era in the Austrian and Spanish armies. References to grenade-throwing troops also appear in England during the Wars of the Three Kingdoms, and it was during the reign of King Louis XIV that companies of soldiers serving as grenadiers were first introduced into the French Royal Army. According to René Chartrand, Jean Martinet formed a grenadier company in the Régiment du Roi in 1667. By 1670 27 French infantry regiments were authorised to include elite companies trained to carry and hurl grenades

The infantry of the Dutch States Army, influenced by their French invaders, adopted grenadiers in 1672. By 1678 six men in each company were trained to throw hand grenades, developed by the Dutch master fireworker Johan van Haren.

In May 1677, the English Army ordered that two soldiers of every Guards Regiment were to be trained as grenadiers; in April 1678 it was ordered that a company of grenadiers be added to the senior eight regiments of foot of the army. On 29 June of that year the diarist John Evelyn saw them drilling at an encampment at Hounslow, near London:

Now were brought into service a new sort of soldier called Grenadiers, who were dexterous in flinging hand grenadoes, every one having a pouch full; they had furred caps with coped crowns like Janizaries, which made them look very fierce, and some had long hoods hanging down behind, as we picture fools. Their clothing being likewise piebald, yellow and red.

The first grenades were small iron spheres filled with gunpowder fused with a length of slow-match, and roughly the size of a tennis ball. The grenadiers had to be tall and strong enough to hurl these heavy objects far enough so as not to harm themselves or their comrades, and disciplined enough to stand at the forefront of the fight, light the fuse and throw at the appropriate moment to minimize the ability of an enemy to throw the grenade back. Understandably, such requirements led to grenadiers being regarded as an elite fighting force.

The wide hats with broad brims characteristic of infantry during the late 17th century were in some armies discarded and replaced with caps. This was originally to allow the grenadier to sling his musket over his back with greater ease while throwing grenades (initially, only these troops were provided with slings). It has been suggested that a brimless hat also permitted the grenadier greater ease in throwing the grenade overhand. Preben Kannik, former Curator of the Danish Army Museum, however, definitely states that grenades were thrown underhand and that it was the slinging of firearms that required the special headdresses typical of early grenadiers.