Fort Valley is a mountain valley located primarily in Shenandoah County, Virginia. It is often called "valley within a valley" as it lies between the two arms of the northern part of the Blue Ridge Mountain range in the Shenandoah Valley in the Ridge-and-Valley Appalachians geological zone.

The eastern ridgeline of Fort Valley is named Massanutten Mountain and is continuous for the eastern length of Fort Valley, with a short stretch called Waterfall Mountain in the extreme south of the valley. The western ridgeline of the valley, also part of the greater Massanutten range, is composed primarily of three named mountains, from north to south: Three Top Mountain, Powell Mountain and Short Mountain. Additionally, behind Three Top and Powell mountains another ridgeline in the chain is called Green Mountain, which is actually higher in elevation than the 3 westernmost named mountains and between which is a smaller highland valley, called Little Fort Valley. The far southern end of Fort Valley also contains a small highland sub-valley called Crisman Hollow, between Catback and Kerns mountains. Duncan's Knob at 2,800 feet, on Catback Mountain, and Strickler's Knob at over 2,700 feet on Masanutten Mountain are the two highest points in the Fort Valley ridgeline. In addition, there are several other named smaller mountains, knobs or notches within the ridges of Fort Valley, and several minor gaps or lower points between the mountains accessible only by hiking trails.

Fort Valley is almost entirely enclosed. At its southern end it is enclosed with an access point named Moreland Gap, and to the north it is constricted and nearly closed but for where Passage Creek flows through a narrow gap providing passage towards Front Royal. Passage Creek also is a prominent element of Fort Valley, running the entire length of the valley at its center, starting in Crisman Hollow in the mountain in the southernmost valley and flowing north, fed by other streams and runs, until it exits Fort Valley just past Elizabeth Furnace. The valley opens out toward the center, becoming about three miles (4.8 km) wide at its widest. The extreme southern end of the valley, the highlands of Crisman Hollow, is in Page County. The remainder of Fort Valley is in Shenandoah County, with the border of Shenandoah and Page Counties being along the eastern ridgeline of Masanutten Mountain. In all, Fort Valley is 23 miles (37 km) long.

Fort Valley is entirely rural, consisting mostly of private farmland, surrounded by the George Washington National Forest (GWNF), which covers the slopes and ridges on both the east and west mountains, offering many outdoor recreation opportunities. The population of Fort Valley is 1,259 with a land area of approximately 70 square miles. The Elizabeth Furnace and Camp Roosevelt Recreation Area of the GWNF are located within Fort Valley.

The Fort Valley and Massanutten mountain range is the result of the Alleghanian Orogeny, the geologic process where a continental plate crumples and is pushed upwards to form mountain ranges. The Fort Valley is an enclosed synclinorium, a downward fold within the Massanutten range. Differential weathering produced the mountain/valley pattern within the Appalachian Ridge and Valley province to which Fort Valley, the Massanutten range and the adjacent Shenandoah and Page Valleys of Virginia belong. The valleys are limestones and flysch (shale + graywacke), susceptible to erosion. Over millennia, the Fort Valley was etched away due to low resistance of its limestone, while the mountain ridges of the Massanutten range are a tough quartz sandstone (Massanutten Sandstone, correlative to the Tuscarora Sandstone further to the west in the Ridge and Valley region) more resistant to that erosion. Thus, the geologic floor of Fort Valley is primarily limestone, with flysch shale, siltstone and some bentonite. A defining feature of the Fort Valley is Passage Creek, now a small stream at the center of the valley but a remnant of a larger water force that carved out the Fort Valley as it appears today.

The Tuscarora trail, a 252-mile long distance hiking trail that passes through Virginia, West Virginia, Maryland, Pennsylvania, proceeds along the northeast ridgeline of Fort Valley, entering from just above Shenandoah State Park on the eastern ridgeline of Massanutten Mountain. The hiking trail passes through the Elizabeth Furnace Recreation Area, ascends Green Mountain and exits on Doll's Ridge on the western slope of Three Top Mountain as it descends towards the Tom's Brook Battlefield in the Shenandoah Valley.

The Massanutten Trail circumnavigates the entire ridgeline of the Fort Valley, 71 miles in total. It was built by the Potomac Appalachian Trail Club (PATC), completed in 2001. Prior to 2001, different portions of the trail had been known by different names and blazed in different colors. With the completion of the missing links, the Forest Service renamed the combined trails as the Massanutten Trail and provided orange blazes along the trail's entire length around Fort Valley.

The Woodstock Tower, built in 1935 by the Civilian Conservation Corps, is located on Powell Mountain, perched at 2,000 feet above Woodstock in the west and the floor of Fort Valley below to the east. Woodstock Tower is accessed via State Route 758 (Woodstock Tower Road), a gravel road with several challenging switchback turns on the western slope, or via hiking the Massanutten Trail which passes directly by the tower. The tower is a sightseeing attraction closely associated with Fort Valley, providing sweeping panoramic views of Shenandoah Valley and the seven bends of the North Fork of the Shenandoah River in the west, and the other Massanutten ridges and Page Valley to the east.