Foreign support in the Winter War consisted of materiel, men and moral support to the Finnish struggle against the Soviet Union in the Winter War. World opinion at large supported the Finnish cause. The Second World War had not yet begun in earnest and was known to the public as the Phoney War; at that time, the Winter War saw the only real fighting in Europe besides the German and Soviet invasion of Poland, and thus held major world interest. The Soviet aggression was generally deemed unjustified. Various foreign organizations sent material aid, such as medical supplies. Finnish immigrants in the United States and Canada returned home, and many volunteers (one of them future actor Christopher Lee) traveled to Finland to join Finland's forces: 8,700 Swedes, 1,010 Danes (including Christian Frederik von Schalburg, a captain in the Danish Royal Life Guards and later commander of the Free Corps Denmark, a volunteer unit created by Nazi Germany in Denmark during World War II), about 1,000 Estonians, 725 Norwegians, 372 Ingrians, 366 Hungarians, 346 Finnish expatriates, 4 Latvians, 2 Lithuanians and 190 volunteers of other nationalities made it to Finland before the war was over. 100 Greek volunteers also volunteered, but did not reach Finland in time.Kaurinkoski, K. (2009). Kreikka ja Suomen talvisota: Sodan kuva Kreikan lehdistössä (1939–1940). In A. Holmila (Ed.), Talvisota muiden silmin: Maailman lehdistö ja Suomen taistelu (pp. 102–124). Atena.

Pope Pius XII condemned the Soviet attack on 26 December 1939 in a speech at the Vatican and later donated a signed and sealed prayer on behalf of Finland.

Belgium There was widespread popular support for Finland in Belgium which, at the time, was a neutral state under the "policy of independence". 52 Belgians served as volunteers in the Finnish army.

Canada 250 Canadians of Finnish descent volunteered to fight for Finland. They fought as part of the Finnish American Legion. Initially the Canadian Foreign Enlistment Act was a hindrance for Canadian citizens who also wished to volunteer. However on 1 March 1940, Canada announced that its citizens were free to enlist in the Finnish Armed Forces. An additional 2,000 Canadians volunteered to fight for Finland as part of a unit known as "Finland’s Little Army".

Denmark

1,010 Danes volunteered to fight for Finland. Most were stationed in Oulu and never saw combat. An exception were Danish pilots that were stationed at various air bases. Among these were two marine pilots that deserted to join Finland, one of which was killed after having shot down four Soviet planes. A total of 4 Danes were killed - 3 of them pilots.

Estonia Estonia remained officially neutral, but 400–1000 Estonian individuals travelled clandestinely to Finland and volunteered to fight against the Soviet invaders. It is estimated that around 100 Estonian volunteers on the Finnish side saw military action before the end of the Winter War. First Estonian citizens, who joined the Winter War were the Ingrian Finns living in the eastern part of Estonia. Johannes Valtonen, who died on the 9th of February is considered to be the first Estonian citizen to die in World War II.

Greece The war aroused great interest in Greece; a popular movement emerged to help Finland, considering it to be the 'heroic nation of the North'. In Greek press, the war was depicted as a battle of David against Goliath and Finland was depicted extremely positively throughout.Koivuniemi, W. (2020). Daavidin ja Goljatin taistelut Euroopan vastakkaisilla laidoilla: Italian-Kreikan sodan ja talvisodan väliset yhtäläisyydet ja niiden uutisointi suomalaisessa sanomalehdistössä [Master’s thesis, University of Helsinki]. HELDA – University of Helsinki Repository.

Although the Greek government took a neutral stance in the war, it tacitly approved aid activities for Finland. They were carried out under the Hellenic Athletics Federation and thousands of people signed up for donations. Over 50,000€ were collected in the first fundraising campaign; the Finnish Red Cross also received loads of donations.

In addition to money, material goods were also donated. Two million cigarettes, wine, figs, raisins and 1,200 boxes of oranges were sent to Finland. The Finnish Consulate General's annual report in 1940 stated how "despite the economic depression, widespread unemployment, and difficult conditions, people responded to the appeals for donations and willingly contributed their share for the good of Finland".

In addition, about 100 Greek volunteers registered with the Finnish Consulate General in Piraeus to fight on Finland's side, but the Greek government considered their deployment undesirable because of fears of Soviet reprisals against the large Greek minority in the Soviet Union. Nevertheless, most of them still sought to go despite their government's position. Several Greek pilots were ready to depart when the Moscow Peace Treaty was announced on 12 March 1940, bringing the Winter War to an end before they could reach Finland.

Hungary

The Hungarian government officially did not support Finland, but secretly started to search for ways to help. In addition, non-governmental organisations began to organize support for Finland. Hungary helped Finland by giving monetary donations, armaments and military volunteers. Nobel Prize winner Albert Szent-Györgyi offered all of his prize money to Finland.

Count Pál Teleki's government sent armaments and war equipment valued at 1 million Hungarian pengős during the Winter War (with knowledge and accord of Regent Miklós Horthy de Nagybánya). The recruiting of volunteers started on 16 December. During the Winter War, around 25,000 Hungarian men applied to fight in Finland; finally, 350 applications were accepted. Their military training started at 10 January and it took almost a month. The volunteers formed a battalion what was commanded by Lieutenant Imre Kémeri Nagy. The Hungarian Volunteer Detached Battalion had 24 officers, 52 non-commissioned officers, 2 physicians and 2 military chaplains for a total of 346 officers and men.

Travel to Finland was very difficult as the German Reich forbade transit of armaments and war equipment across its territory (including the occupied Polish territories). Therefore, volunteers had to travel across Yugoslavia, Italy, France, the United Kingdom, Norway and Sweden to make their ways to Finland. They travelled without any weapons by a special train, officially classified as "tourists going to ski-camp". Finally the battalion arrived in Finland at 2 March after 3 weeks travelling.

In Finland the battalion was quartered in Lapua, in the training center of the international volunteers. In Lapua they took a part in further military training, learning skiing and winter warfare. Before the Hungarian battalion could see military action, the Moscow Peace Treaty was signed, on 12 March in Moscow, leaving many volunteers frustrated.

In the last days of March, Field Marshal Mannerheim visited Lapua where he met the Hungarian battalion. He expressed his thanks to the volunteers for coming to Finland and he promoted Lieutenant Imre Kémeri Nagy to Captain. From 17 April to 19 May, the Hungarian battalion served in Karelia, at the new state border in Lappeenranta.

The Hungarian battalion was embarked at Turku at 20 May 1940, from where a steamboat sailed to Stettin, German Reich (now Szczecin, in Poland). They traveled across the German Reich by a special train with a German guard. The volunteers arrived at Budapest on 28 May.

Besides the Hungarian Volunteer Detached Battalion, other Hungarian volunteers fought in the Winter War in the Finnish army, travelling to Finland individually. 2nd Lieutenant Mátyás Pirityi served in the Finnish Air Force and took part in more than 20 sorties. Warrant Officer Vilmos Békássy's plane disappeared over the Gulf of Bothnia. Géza Szepessy, along with four fellows from the Military Technical College of Berlin, went to Finland where he was wounded in action.

Italy In violation of the Italo-Soviet Pact, Fascist Italy had staunchly supported Francisco Franco during the Spanish Civil War in his fight against the Second Spanish Republic, which was supported by the Soviet Union in a proxy war. Italy violated the pact for the second time by promptly responding to requests by the Republic of Finland for military assistance and equipment for use against the Soviet government. The Royal Italian Air Force (Regia Aeronautica Italiana) sent thirty-five Fiat G.50 fighters, while the Royal Italian Army (Regio Esercito Italiano) supplied 94,500 new M1938 7.35 mm rifles for use by Finnish infantry. However, the Soviet Union's new partner Germany intercepted much of Italy's aid. 32 planes still made it to Finland, mostly via sea, before the war was over.Kari Stenman and Kalevi Keskinen, Finnish Aces of World War 2, Oxford, Osprey Publishing, 2012,pp.12-13

A handful of Italian volunteers also fought in the Winter War on the side of Finland.

Norway

The Norwegian government prohibited officers or under-officers to volunteer for the war in Finland out of fear that this would aggravate the Germans (they wanted to remain neutral at all costs). Of the 725 Norwegians that volunteered to fight for Finland, only 125 made it to the relatively tranquil Salla front and then just three weeks before the war ended. Several of the future leaders of the Norwegian resistance movement such as Max Manus and Leif "Shetland" Larsen were among the volunteers. The most highly decorated Norwegian in the later resistance movement, Gunnar Sønsteby, spent his stay as an office clerk (like many of his countrymen in the Winter War).

In addition to the military volunteers, 30 physicians and 40 nurses went to help the Finnish medical system, under the auspices of the Norwegian People's Aid.

There were numerous nationwide collection campaigns of supplies and money in Norway to help the Finns. In all, the Finland collection () brought in some , the largest popular collection in Norwegian history. Six training aircraft were purchased for part of these funds. Initial flying training was given, close to Oslo, with these aircraft to students sent from Finland.

An important venue for collections for Finland were sporting events, several of which were held for the benefit of Finland in Norway during the war. Some 50,000 backpacks filled with supplies were collected in Norway and dispatched to Finland. Collections of rifles (mostly Krag–Jørgensen models) and home knitted shooting gloves also took place. Sigrid Undset, Norwegian author and Nobel laureate, donated her Nobel medal to Finland on 25 January 1940.Finnish Defence Forces official website: The Winter War – Day 57

The Norwegian government secretly donated to the Finns 12 German-made 7.5 cm field gun m/01s (designated 75 K 01 in Finnish service) in February 1940.Winterwar.com: Finnish Artillery pieces – 75 K 01Winterwar.com: Artillery acquisitions during the Winter War Included in the covert artillery transfer were 12,000 shells. Norway also allowed the transfer of aircraft to Finland via Sola Air Station, near Stavanger.Fighter Tactics Academy: Brewsters to the Finnish Air Force in 1940 Fighter Tactics Academy: Hurricanes to Finland Secret Mission To Fly Twelve Bristol Blenheims To Finland BBC – World War 2 People's War Norwegian volunteers took part in the assembly of some of the aircraft at the Saab factory in Trollhättan, Sweden.

One of the main reasons that the Franco-British plan—Operation Avon Head—to send troops to Finland never materialized was that Norway would not allow them to use their ports and territory for troops transfer in order to preserve a policy of neutrality. They explicitly threatened to fire upon any ship that came near Trondheim or Narvik on that mission, as it was correctly assumed that such a force would in reality be used to occupy those towns.