I think the closest would be the Rape of Nanking, when the Japanese killed hundreds of thousands of Chinese, but beyond that the Japanese had moved into the main Asian Continent in the 1931 when they invaded Manchuria, so it wasn't a single event that occured in World War II, but an entirely separate war that got dwarfed by WWII. They called it the War of Resistance in China, but that is marked as beginning in 1937.

These days they tend to refer to it (in English) as the "Anti-Japanese War" or "War of Anti-Japanese resistance" rather than just "War of Resistance". I don't know how far back that goes though, and of course it's a translation anyway.

But the Japanese conquests of Korea and Manchuria, and their invasion of China, were not a deliberate attempt to wipe out particular ethnic groups like the Holocaust was. It was a more conventional war, that simply had a massive death toll, the exception being the Rape of Nanjing, which does have a particular name.

Incidentally, on the Rape of Nanjing, if you are ever in Nanjing, there is an excellent memorial museum that I strongly recommend. The numbers are possibly a little exaggerated, but there is definitely a lot of strong primary evidence presented, and (unlike a lot of Chinese anti-Japanese propaganda) it seemed fairly reliable.