The Bastrop County Complex fire was a conflagration that engulfed parts of Bastrop County, Texas, in September and October 2011. The wildfire was the costliest and most destructive wildfire in Texas history and among the costliest in U.S. history, destroying 1,696 structures and causing an estimated $350 million in insured property damage. An exceptional drought, accompanied by record-high temperatures, affected Texas for much of 2011. Vegetation consequently became severely parched throughout the state, and over the year an unprecedented amount of land in the state was burned by numerous wildfires. In early September 2011, the presence of Tropical Storm Lee to the east produced strong northerly winds over the state, exacerbating the preexisting dry weather to produce critical fire conditions. On the afternoon of September 4, 2011, three separate fires ignited in the wildland–urban interface east of Bastrop, Texas, after strong winds caused by the nearby tropical storm snapped trees onto power lines. Within 48 hours, the fires merged into one blaze that quickly consumed parts of Bastrop State Park and parts of the Lost Pines Forest, as well as homes in nearby subdivisions. Most of the conflagration's spread and destruction occurred within a week of ignition, as the forward advance of the wildfire mostly stopped after September 7. The wildfire was largely contained in September, though the firebreak was briefly breached in early October. On October 10, the Bastrop County Complex was declared controlled, and the fire was declared extinguished on October 29 after 55 days of burning within the fire perimeter.

Two people were killed by the wildfire, and another twelve people were injured. The fire perimeter encircled an area spanning at least 32,000 acres (12,950 hectares). Homes were destroyed in ten subdivisions, of which Circle-D County Acres and Tahitian Village sustained the most significant property damage. The wildfire destroyed more homes than any single fire in Texas history by nearly a factor of ten. Around 96 percent of Bastrop State Park was scorched by the wildfire. Roughly 1.5 million trees across 16,200 acres (6,600 hectares) of forest were either killed directly by the fire or fatally damaged. Despite the severe habitat loss, populations of local herpetofauna were not significantly affected by the fire, though extensive soil erosion was enabled by the loss of groundcover.

The Bastrop County complex occurred within the Lost Pines Forest of Texas, a region characterized by a loblolly pine (Pinus taeda) canopy, a yaupon (Ilex vomitoria) understory, sandy soils, and a mixed topography of flat terrain and rolling hills. This area is near the city of Bastrop, Texas, which itself is located about 30 mi (48 km) southeast of Austin, the state capital of Texas. Oak trees, Ashe juniper (Juniperus ashei), shrubs, and grasses also comprise the vegetation of the region. The Lost Pines cover a 70 sq mi (180 km²) area and are a disjunct population separated from the Piney Woods by 100 mi (160 km) of agricultural land. Historical wildfire behavior in Texas has been strongly influenced by human activities and changes in land use, affecting the makeup and composition of the vegetation impacted by the fire. The prevalence of cattle in Bastrop County between the 1860s and mid-1880s may have led to a reduction in fires in and around what is now Bastrop State Park via the grazing of potential fuels. However, the harsh winter of 1886–1887 brought an end to open-range grazing by 1890; this may have allowed for an increase in wildfires in subsequent decades. Greater utilization of wildfire suppression after the mid-1940s led to an increase in tree density in the region, further augmented by the concurrent planting of additional loblolly pine trees. The reduction in wildfires after the 1940s – leading to the lowest fire activity since at least 1720 – led to a build-up in vegetation density (and thus potential fuels for a wildfire) to unprecedented levels. In the years leading up to the Bastrop County Complex, the only large wildfire in the area was the Wilderness Ridge Fire in 2009, which burned 1,491 acres (603 hectares). The vegetation in and around Bastrop State Park indicates that no fire before 2011, dating back to at least 1650, matched the severity of the Bastrop County Complex.

Texas endured one of its worst droughts in recorded history throughout much of 2011. The drought began to materialize following a drier than average autumn and winter beginning in 2010, but it worsened to widespread and extreme levels after March 2011. The average precipitation total statewide was 0.29 in (7.4 mm) compared to the 1981–2010 average of 2.18 in (55 mm) in March, making it the state's driest March on record. The period from October 2010 to September 2011 was the driest 12-month period in Texas history, with the statewide average rainfall falling below the previous record set during the 1950s Texas drought by 2.35 in (60 mm). Based on paleoclimate reconstructions, the summer of 2011 may have been the fourth driest summer in Texas since 996. Temperatures during the summer of 2011 also rose to record highs, with statewide average temperatures from June through August 2011 eclipsing the previous record high by 2 °F; each of the three months was their respective warmest month on record. Additionally, the mean statewide temperature in those three months were the hottest summer months recorded in any U.S. state on record, topping the record heat set in Oklahoma during the Dust Bowl.

Dry conditions perpetuated by the drought led to widespread die-offs of trees across central and eastern Texas by the early fall of 2011, and most forests throughout the state were exceptionally dry; 251 of the 254 counties in Texas had bans on outdoor burning in effect in September 2011. The moisture content of all vegetation types within Bastrop County diminished to record lows during the course of the drought, making them increasingly susceptible to combustion. According to the U.S. Drought Monitor, the entirety of Bastrop County had remained in exceptional drought – the most severe drought conditions – since the week of May 10, 2011, and had been experiencing at least abnormally dry conditions since at least the week of October 26, 2010. Numerous wildfires occurred across Texas and adjoining states in 2011; more land area was burned in Texas and Oklahoma since official recordkeeping began in 2002. Between November 2010 and September 2011, Texas wildfires engulfed over 3.7 million acres (1.5 million hectares) of land; within the first week of September 2011, 135,000 acres (54,600 hectares) burned.

Amid the exceptional drought in early September 2011, Tropical Storm Lee developed in the Gulf of Mexico and produced northeasterly winds across much of East Texas beginning on September 3; the storm itself was centered over the gulf south of the central Louisiana coast. These winds were further accelerated by the presence of high air pressure over Texas. Temperatures ahead of a cold front concurrently moving east of the Rocky Mountains rose above 100 °F (38 °C) the day before the fire ignited and on the day of ignition, resulting in relative humidity values falling below 20 percent in the vicinity of Bastrop. Rainfall associated with the tropical storm did not expand west of Interstate 45, allowing the dry conditions in the Bastrop area to persist. Winds decreased over the night of September 3 and into the morning of September 4, but the approach of the cold front into the Bastrop area resulted in the acceleration of winds oncemore. Around the time of the Bastrop County Complex's initiation on September 4, the distant tropical storm was generating sustained winds of 12–14 mph (19–23 km/h) and wind gusts of 25–31 mph (40–50 km/h) in the Bastrop area. The high temperature on September 4 was 101 °F (38 °C), and the relative humidity bottomed out at 20 percent. This combination of conditions led the Storm Prediction Center to forecast critical fire weather conditions over southern and central Texas for September 4. Throughout Texas, local fire departments responded to 227 fires on September 4, 57 of which were new fires. The likelihood of a wildfire igniting given the prevailing weather conditions was around 90 percent.

The wildfire complex began as three individual fires whose ignitions were reported within three hours on September 4, 2011. The first fire was ignited by fallen power lines near Circle D-KC Estates, Texas, located 16 mi (10 km) northeast of Bastrop. A nearby homeowner called 9-1-1 at 2:20 p.m. CDT, reporting the fire. An investigation performed by the Texas Forest Service (TFS) determined a point of origin near 258 Charolais Drive. Strong winds caused a pine tree to snap 8 ft (2.4 m) above the ground, resulting in the upper trunk falling atop power lines and triggering sparks that reached the dry grass and leaf litter below. The first fire grew quickly after ignition; the Bastrop volunteer fire department requested fire suppression equipment from the TFS at 2:25 p.m. CDT but determined at 2:33 p.m. CDT that the blaze was uncontrollable, prompting evacuations. Thirty-two minutes later after the first ignition, a second fire initiated 4 mi (6.4 km) to the north near Schwantz Ranch Road and U.S. Highway 290; the TFS determined that this second ignition was also caused by a tree falling upon power lines. An investigation by Bluebonnet Electric Cooperative, which services the power lines in the region, arrived at similar conclusions for the cause of the fires. Pushed southward by a strong northerly wind, the wildfire crossed Texas State Highway 21 at 3:02 p.m. CDT. Mike Fisher, the emergency management coordinator for Bastrop County, formally declared the situation a disaster at that time, authorizing aid from outside of the county. The conflagration subsequently spread into Bastrop State Park and began to encroach upon Texas State Highway 71. The highway's right-of-way formed a preexisting firebreak spanning 325 ft (99 m) across. Firefighters set dead grass in the highway's median strip to deter expansion of the flames south of the highway, but were unsuccessful; the first fire crossed the highway at 4:07 p.m. CDT. The two initial fires grew and aggregated into a single wildfire by 5 p.m. CDT after joining near Cardinal Drive, continuing to spread through Bastrop State Park and across Texas State Highway 71.

The third individual fire was ignited by a tree contacting a power line south of the highway near Tahitian Drive at 5:16 p.m. and eventually merged with the larger fire; the complex of wildfires merged into a single wildfire within 48 hours of ignition. During the first hours of the wildfire, the firefront advanced at 5 mph (8.0 km/h) through the pine and yaupon vegetation. High winds carried hot embers far from the firefront, igniting fires as far as 3 mi (4.8 km) away. Fire whirls were also observed along the firefront. In addition to the strong winds, horizontal convective rolls embedded within the wind flow enabled the wildfire to quickly advance along the tops of the forest canopy, resulting in long streaks of intensely burned vegetation. By the end of September 4, the combined wildfire had scorched an area roughly 14 mi (23 km) long and up to 6 mi (9.7 km) wide, covering 14,000 acres (5,660 hectares) of land.