The Battle of Kollaa was fought from December 7, 1939, to March 13, 1940, in Ladoga Karelia, Finland, as a part of the Soviet-Finnish Winter War.

After a string of defeats incurred by the 26th Finnish regiment, the 24th regiment entrenched themselves west of the Kollaa River. The Soviet thrust against this sector was largely unexpected, and opened grave possibilities of Red forces outflanking the Finnish line of defence north of Lake Ladoga and bypassing the Mannerheim Line. Hence, large portions of the Finnish Fourth Corps were diverted to the attack. Despite still having far fewer troops than the Soviets, the Finnish forces (12th Division) repelled the Red Army because the Soviets were only prepared to proceed along roads. With few roads in the Kollaa area, and all of them guarded by Finnish troops, the Soviets were unable to proceed cross-country without skis.

Kollaa is considered to have been one of the most difficult locations to defend during the Winter War. The creek-sized Kollaa River was surrounded by soil so cold in the winter months that the ground was nearly impossible to dig in.

It is estimated that the Red Army fired nearly 40,000 artillery rounds at the defence line in a single day. In contrast, the Finnish artillery could fire a maximum of 1,000 rounds per day. By the end of the battle, the ground was all but pulverized into slush. It was in this battle and surrounding ones that a Finnish war train distinguished itself, becoming iconic within the forces and raising morale at every loud appearance. The Soviet attacks were for the most part frontal assaults led by long columns of tanks followed by infantry. Many of these pushes were quickly disbanded after the leading tanks were disabled by anti-tank guns, which forced them into retreat.

One defence point, called "Killer Hill" by the Finns, saw the Soviets advance an entire regiment against a force of 32 fortified Finns. Four-hundred Soviets died, along with 28 of the defenders.

The Battle of Kollaa continued until the end of the Winter War, despite the Finnish 12th Division stopping the 8th Red Army and both sides suffering heavy losses. The Red Army managed to penetrate the Finnish defence line in Kollaa several times, pushing the Finns out of their positions. The Finns restored the integrity of their defence line through systematic counter-attacks. On March 12, near the end of the war, the Soviets managed to form a 0.5–1.5 kilometres (0.31–0.93 mi) deep fracture point in the Finnish defence line, nearly resulting in its collapse. As a result, the commander of the Finnish Army's 12th Division considered abandoning the main defence line at Kollaa. However, as the news from the sector was that the situation was "not yet that alarming", the commander ordered a counter-attack, for the defence line to be retaken the following day. These orders were rescinded, as news of the concluded peace treaty reached the front, and the men were ordered to hold their current positions until the end of hostilities.

A famous quote from the Battle of Kollaa is Major General Hägglund's question, "Will Kollaa hold? (Kestääkö Kollaa?)", to which Lieutenant Aarne Juutilainen replied, "Kollaa will hold (Kollaa kestää), unless the orders are to run away." The simple question and reply have entered the Finnish lexicon as an expression of perseverance and resolve in the face of impending difficulty or crisis. The Finnish punk band Kollaa Kestää [fi] uses the name.

The legendary Finnish sniper Simo Häyhä, nicknamed the "White Death", would see his first battle on the Kollaa front. He is credited with at least 505 confirmed kills during the war, according to Finnish military records.

• List of Finnish military equipment of World War II
• List of Soviet Union military equipment of World War II