A ranger, park ranger, park warden, field ranger, or forest ranger is a person entrusted with protecting and preserving parklands and protected areas – private, national, state, provincial, or local parks. Their duties include (but are not limited to) law enforcement, wildlife and land management, community engagement and education, recreation area maintenance, and firefighting. Rangers monitor wildlife, remove snares, confront and arrest poachers, identify and remove invasive species, and much more.

"Parks" in this context may be broadly defined by some systems in this context and may include forests, wildlife preserves, deserts, beaches, and even protected culturally/historically important manmade environments and monuments. Park rangers are not limited to working in the natural environment.

Different countries use different names for this occupation. Warden is the favored term in Canada, Ireland, the United Kingdom, and many other Commonwealth countries. The profession includes a number of disciplines and specializations, and park rangers are often required to be proficient in more than one.

The nomenclature is more complex in the United States. The National Park Service refers to the position as Park Ranger for all law enforcement, interpretive, and fee collection booth rangers. Similarly, within the Bureau of Land Management, interpretive staff members are called Park Rangers while law enforcement staff are referred to as LE Rangers. District Rangers in the U.S. Forest Service do not perform the same duties, instead focusing on administration/management of “Ranger Districts”, large subunits of National Forests. Few other positions in the U.S. Forest Service are officially titled "Ranger", but there are several positions that perform similar or identical outdoor work including those with the titles Forester, Forestry Technician, and Law Enforcement Officer. Many are referred to colloquially as "rangers" such as Wilderness Rangers, Climbing Rangers, Snow Rangers, Recreation Rangers, and Forest Protection Officers even though they are all officially categorized as Forestry Technicians.

In medieval England, rangers, originally called under-foresters, were the most junior officials employed to "range" through the countryside enforcing the forest law imposed by William the Conqueror to protect the "vert and venison". Their duties were originally confined to seeing that the Forest Law was enforced in the borders, or purlieus, of the royal forests. Above them were the Foresters-in-Fee (later called Woodwards), then the Verderers, then the Justices in Eyre. Their duties corresponded in some respects with that of a mounted forester.

The term ranger seems to correspond to the Medieval Latin word regardatores which appeared in 1217 in the Charter of the Forest. Regardatores was later rendered as rangers in the English translations of the Charter. Others translate it as regarders. For example, the fifth clause of the Charter of the Forest is commonly translated: "Our regarders shall go through the forests making the regard as it used to be made at the time of the first coronation of the aforesaid King Henry [II] our grandfather, and not otherwise." A "regard" is considered to be an inspection of the forest.

The earliest letters patent found mentioning the term refer to a commission of a ranger in 1341. Documents from 1455 state that England had "all manner and singular Offices of Foresters and Rangers of our said Forests".

One of the first appearances of ranger in literature is in Edmund Spenser's poem The Shepheardes Calendar from 1579: "[Wolves] walk not widely, as they were wont, for fear of rangers and the great hunt."