The Battle of Island Number Ten was an engagement at the New Madrid or Kentucky Bend on the Mississippi River – forming the border between Missouri and Tennessee – during the American Civil War, lasting from February 28 to April 8, 1862. Island Number Ten, a small island at the (Tennessee) base of a tight double turn in the river, was held by the Confederates from the early days of the war. It was an excellent site to impede Union efforts to invade the South by the river, as ships had to approach the island bows on and then slow to make the turns. For the defenders, however, it had an innate weakness in that it depended on a single road for supplies and reinforcements. If an enemy force managed to cut that road, the garrison would be isolated and eventually be forced to surrender.

Union forces began the siege in March 1862, shortly after the Confederate Army abandoned their position at Columbus, Kentucky. The Union Army of the Mississippi under Brigadier General John Pope made the first probes, coming overland through Missouri and occupying the town of Point Pleasant, Missouri, almost directly west of the island and south of New Madrid. Pope's army then moved north and soon brought siege guns to bear on New Madrid. The Confederate commander, Brig. Gen. John P. McCown, decided to evacuate the town after only one day of heavy bombardment, moving most of his troops to Island No. 10, abandoning his heavy artillery and most of his supplies.

Two days after the fall of New Madrid, Union gunboats and mortar rafts sailed downstream to attack Island No. 10. Over the next three weeks, the island's defenders and forces in the nearby supporting batteries were subjected to a steady bombardment by the flotilla, mostly carried out by the mortars. At the same time, the Union forces at New Madrid were digging a canal across the neck of land east of the town to bypass Island No. 10. Several transports were sent to the Army of the Mississippi when the canal was finished, which provided the army with a way to cross the river and attack the Confederate troops on the Tennessee side.

Pope persuaded Flag Officer Andrew Hull Foote to send a gunboat past the batteries, to assist him in crossing the river by keeping off any Southern gunboats and suppressing Confederate artillery fire at the point of attack. The USS Carondelet, under Commander Henry Walke, slipped past the island on the night of April 4, 1862. This was followed by the USS Pittsburg, under Lieutenant Egbert Thompson two nights later. With the support of these two gunboats, Pope was able to move his army across the river and trap the Confederates opposite the island, who by now were trying to retreat. Outnumbered at least three to one, the Confederates realized their situation was hopeless and decided to surrender. At about the same time, the garrison on the island surrendered to Flag Officer Foote and the Union flotilla.

The Union victory marked the first time the Confederate Army lost a position on the Mississippi River in battle. The river was now open to the Union Navy as far as Fort Pillow, a short distance above Memphis. Only three weeks later, New Orleans fell to a Union fleet led by David G. Farragut, and the Confederacy was in danger of being cut in two along the line of the river.

Island No. 10 owed its name to the fact that it was at one time the tenth island in the Mississippi River south of its junction with the Ohio. An evanescent product of the river, it was an enlarged sandbar, roughly 1 mi (1.6 km) long and 450 yd (410 m) wide at its maximum width, and standing about 10 ft (3.0 m) above low water.

More important than the island itself was the course of the river in its neighborhood. Island No. 10 was at the southern extremity of a clockwise turn of the river through 180°, which was followed immediately by a counterclockwise turn that left the river moving almost parallel to its original course, but displaced to the west by about 8 mi (13 km). The turns are quite tight: the distance from the southern limit of the first turn to the northern limit of the second is only 9 mi (14 km) by air, or 12 mi (19 km) measured along the river channel. The double bend, which still exists in almost the same location, is known as the New Madrid Bend. However, the area across the Mississippi River from New Madrid, Missouri on the Kentucky and Tennessee shore was known during the Civil War simply as Madrid Bend. The town of New Madrid (pronounced MAD-rid), which gives the bend its name, is at the northern apex of the second turn.

The mainland behind the island on the south side was connected to the town of Tiptonville, Tennessee, by a good road on the natural levee of the river. This was the only approach to the island on dry land through Tennessee, as the region is otherwise a mixture of lakes, sloughs, and swamps, with the nearest high ground nearly 10 mi (16 km) to the east. Reelfoot Lake, the largest of these, was 40 mi (64 km) long and in places 10 mi (16 km) wide. In low water, the northern end of Reelfoot Lake was near Tiptonville, but in high water such as was present in the spring of 1862, it extended north to beyond the bend. The water was nowhere very deep, so individual soldiers could cross it by wading or using makeshift rafts, but an army trying to do so would not be able to move its heavy equipment, and also would lose cohesion. For these reasons, Island No. 10 was considered to be invulnerable to land attack from the Tennessee side. It also meant, however, that the only route for either reinforcement or escape was the Tiptonville road.