Korea during the Japanese occupation period (between 1910 and 1945) had various 'independence movements' where various groups attempted to fight against the Japanese. A big portion of these independence movements occurred in what is modern-day China and Primorsky Krai.
Right after the October Revolution, the ideals of the Bolsheviks started to influence the far east, and soon later the independence activists started to get influenced too. By the early 1920s, socialism became widespread across Koreans inside and outside of the Korean peninsula.
Korean socialists of this era(20s~post 1945) claimed the creation of a 'Soviet Korean government and country' and 'the removal of the class system and the bourgeois inside the Korean society'. They wanted the proletarians (the farmers and students of Korea) to create movements directly against Japan and Japanese imperialism. However, not all Korean independence movement leaders agreed on this principle; they wanted to focus more on 'independence first' and 'actually preparing the Korean people to be capable of fighting for independence'. These people are called the Ethnic Nationalists and they went as far as attempting to make a citizen-funded university to educate the Korean people for the fight against the Japanese.
These different ideas between the Ethnic Nationalists and the Socialists were really the beginning of Korea's division, but both sides had a common goal of kicking out the Japanese. But it still created a lot of tensions between Koreans, such as President Rhee Syngman (리승만, later on, becomes the first president of South Korea) and Prime Minister Yi Dong-hwi(이동휘) fighting each other inside the Provisional Government of the Republic of Korea. Rhee wanted an American styled Liberal Democracy but Yi claimed there should be a socialist revolution first.
One of the first groups to form out of communism was the Koryeo Communist Party(고려공산당) in 1921 which was divided up into the Shanghai Faction and the Irkutsk Faction. The Shanghai Faction (which consists of Koreans in China) was the moderate one which claimed 'a National Revolution against the Japanese should come first, and after that, it should be changed into a Proletariat Revolution'. Meanwhile, the radical Irkutsk Faction (made up with Koreans in Siberia) wanted an 'an establishment of a Soviet nation through an immediate Socialist Revolution". Both of them ultimately wanted an independent communist Korea, but due to the difference, they didn't have a close relationship with each other. Even inside the Provisional Government (mentioned above), the two sides fought each other (while also fighting the Ethnic Nationalists), and their division also led to the Svobodny Incident (자유시 참변,自由市慘變) where the two factions were on a dispute on who's the commander of the Independence Army(독립군, 獨立軍) ultimately leading to the army being surrounded and killed by the Russians. Due to the division between these two, in 1922 the Communist International created the Корбюро(Courburo,꾜르뷰로,高麗部) (the agency that deals with Korea related issues in the Comintern) and dissolved the two factions.
After that, various people on the left felt the strong need for a unified party. The Tuesday Group(화요회,火曜會) (is named the 'Tuesday Group' because Karl Marx was born on a Tuesday), which is a socialist academic research group created by several former members of the dissolved two factions and some people who studied abroad in Japan, and various other socialist groups inside of Korea joined together.
On April 17th of 1925, 19 notable socialist leaders like Cho Bong-am(조봉암) and Kim Jae-bong(김재봉) created the Joseon Communist Party. Since the Tuesday Group was in control of this new party, the Communist International had control over it. Comintern approved this party as the legitimate communist party of the Koreans under the 'one nation, one party' principle.
The Joseon Communist Party was a secretive group since most of its activity was inside of Korean peninsula, where the Japanese were trying to remove communists entirely. This party was oppressed by the Japanese/Governor-General of Chosen, repeating being dissolved and re-establishing itself. On the first time that the party got dissolved, they attempted to send young student members of the party to Moscow for the purpose of the expansion of the party, but the Japanese caught them doing it.
This party is the direct predecessor of the Worker's Party of Korea(조선로동당), which is the current political party inside of the Democratic People's Republic of Korea.
Also, Kim Il-sung, the first head of state and the guy who established the Democratic People's Republic of Korea, didn't just randomly gain power in 1945 when tanks of the Soviet Union entered the northern half of Korea. By the late 30's, a lot of Korean independence armies who fought the Japanese with physical battles had disappeared due harsher environments after the Mukden Incident and the Japanese eventually taking over all of Manchuria. Kim Il-sung, who led the Northeast Anti-Japanese United Army, decided to cross into the Korean peninsula (he was active in Southern Manchuria) and invade a place called Pocheonbo(보천보). The Battle of Pocheonbo(보천보 전투) only killed 7 Japanese officers, but it was successful. It was a small battle but a significant one since it's an actual battle that the Koreans won inside of Korea. The name Kim Il-sung spread all across the country and he became a significant communist leader during the later periods of the Korean Independence Movement. He easily got a grip of the new DPRK in 1945 due to his activities before 1945.
Basically, yes communism was a thing in Korea even before 1945. It played a big role in Korean independence from Japan. The North Koreans really like to have pride in this (especially the Kim Il-sung part). However, South Korea ignored most of this part of history due to its anti-communist nature. The current South Korean administration is left-leaning so they recognized some of these socialist activists as national heroes, but before that, it was basically totally ignored or even banned.
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