The Battle of the Tenaru, sometimes called the Battle of the Ilu River or the Battle of Alligator Creek, was a land battle between the Imperial Japanese Army and Allied ground forces that took place on 21 August 1942, on the island of Guadalcanal during the Pacific campaign of World War II. The battle was the first major Japanese land offensive during the Guadalcanal campaign.

In the battle, U.S. Marines, under the overall command of U.S. Major General Alexander Vandegrift, repulsed an assault by the "First Element" of the "Ichiki" Regiment, under the command of Japanese Colonel Kiyonao Ichiki. The Marines were defending the Lunga perimeter that guarded Henderson Field which had been captured by the Allies in landings on Guadalcanal on 7 August. Ichiki's unit was sent to Guadalcanal in response to the Allied landings there with the mission of recapturing the airfield and driving the Allied forces off the island.

Underestimating the strength of Allied forces on Guadalcanal, which at the time numbered about 11,000 personnel, Ichiki's unit conducted a nighttime frontal assault on Marine positions at Alligator Creek on the east side of the Lunga perimeter. Jacob Vouza, a Coastwatcher scout, warned the Americans of the impending attack minutes before Ichiki's assault. The Japanese were defeated with heavy losses. The Marines counterattacked Ichiki's surviving troops after daybreak, killing many more. About 800 of the original 917 of the Ichiki Regiment's First Element died.

The battle was the first of three separate major land offensives by the Japanese in the Guadalcanal campaign. The Japanese realized after Tenaru that Allied forces on Guadalcanal were much greater in number than originally estimated and subsequently sent larger forces to the island in their attempts to retake Henderson Field.

During the Pacific campaign of World War II, on 7 August 1942, U.S. forces landed on Guadalcanal, Tulagi, and Florida Islands in the Solomon Islands. The landings on the islands were meant to deny their use by the Japanese as bases for threatening the supply routes between the U.S. and Australia, and to secure the islands as starting points for a campaign with the eventual goal of isolating the major Japanese base at Rabaul while also supporting the Allied New Guinea campaign. The landings initiated the six-month-long Guadalcanal campaign.

Taking the Japanese by surprise, the Allied landing forces accomplished their initial objectives of securing Tulagi and nearby small islands, as well as an airfield then under construction at Lunga Point on Guadalcanal, by nightfall on 8 August. That night, as the transports unloaded, the Allied warships screening the transports were surprised and defeated by an Imperial fleet of seven cruisers and one destroyer, commanded by Japanese Vice Admiral Gunichi Mikawa. One Australian and three U.S. cruisers were sunk and one other U.S. cruiser and two destroyers were damaged in the Battle of Savo Island. Rear Admiral Richmond K. Turner withdrew all remaining Allied naval forces by the evening of 9 August without unloading all the heavy equipment, provisions, and troops from the transports, although most of the divisional artillery was landed, comprising thirty-two 75 mm and 105 mm howitzers. Only five days' rations were landed.

The Marines ashore on Guadalcanal initially concentrated on forming a defense perimeter around the airfield, moving the landed supplies within the perimeter, and completing construction of the airfield. Major General Alexander Vandegrift placed his 11,000 troops on Guadalcanal in a loose perimeter around the Lunga Point area. In four days of intense effort, the supplies were moved from the landing beach into dispersed dumps within the perimeter. Work began on completing the airfield immediately, mainly using captured Japanese gear. On 12 August, the airfield was named Henderson Field after Major Lofton Henderson, a Marine aviator who had been killed at the Battle of Midway. Captured Japanese stock increased the total supply of food to 14 days' worth. To conserve the limited food supplies, the Allied troops were limited to two meals per day.

In response to the Allied landings on Guadalcanal, the Japanese Imperial General Headquarters assigned the Imperial Japanese Army's 17th Army, a corps-sized command based at Rabaul and under the command of Lieutenant-General Harukichi Hyakutake, with the task of retaking Guadalcanal from Allied forces. The 17th Army, then heavily involved with the Japanese campaign in New Guinea, had only a few units available to send to the southern Solomons area. Of these units, the 35th Infantry Brigade under Major General Kiyotake Kawaguchi was at Palau, the 4th (Aoba) Infantry Regiment under Major General Yumio Nasu was in the Philippines, and the 28th (Ichiki) Infantry Regiment, under the command of Colonel Kiyonao Ichiki, was at sea en route to Japan from Guam. The different units began to move towards Guadalcanal immediately, but Ichiki's regiment, being the closest, arrived first.