Lost Arrow Spire, Granite climbing spire in Yosemite Valley, United States.

Lost Arrow Spire is a detached granite pillar about 80 metres tall, separated from the Yosemite Falls cliff wall in Yosemite Valley. Its rock is extremely compact granite with almost no cracks. A notch at its base, Arrow Notch, separates the spire from the main wall, and the last two pitches, the Lost Arrow Spire Tip, complete the detached portion.

In 1946 Ax Nelson lassoed the tip from the valley rim, 125 feet away, and prusiked up, a stunt Steve Roper called one of the greatest rope stunts in climbing history. In 1947 Nelson and John Salathé made the true first ascent via the Lost Arrow Spire Chimney, taking five days with Salathé's pitons forged from Ford Model A axle alloy. Roper called it the first big-wall climb in the United States.

The name translates as "lost arrow", after the detached pillar's shape. Since 1985 the gap between the tip and the valley rim has been crossed on a slackline about 17 metres long, roughly 880 metres above the valley floor; Scott Balcom made the first crossing on 13 July 1985, and Darrin Carter crossed without a safety leash in 1995.

Access is gained through trails starting from Camp 4 and leading toward Upper Yosemite Falls, where climbers then navigate to the spire itself. The climb requires technical skill and proper equipment, including ropes for managing the descent through the narrow notch section.

The first slackline crossing to the spire was made by Scott Balcom on 13 July 1985, using a safety leash; the line is about 17 metres long and hangs roughly 880 metres above the valley floor. In 1995 Darrin Carter crossed without any leash, and in 2003 Dean Potter did so in both directions.