In golf, a hole in one or hole-in-one (also known as an ace, mostly in American English) occurs when a ball hit from a tee to start a hole finishes in the cup. A ball hit from a tee following a lost ball, out-of-bounds, or water hazard is not a hole-in-one due to the application of a stroke penalty.

Holes-in-one most commonly occur on par 3 holes, the shortest distance holes on a standard size golf course. Longer hitters have also accomplished this feat on longer holes, though nearly all par 4 and par 5 holes are too long for golfers to reach in a single shot. While well known outside golf and often requiring a well hit shot and significant power, holes in one need also a significant element of luck. As such, they are more common and considered less impressive than other hole accomplishments such as completing a par 5 in two shots (an albatross). As of January 2021, a condor (four under par) hole-in-one on a par 5 hole had been recorded on five occasions, aided by thin air at high altitude, or by cutting the corner on a doglegged or horseshoe-shaped hole.