Technically Finland is not a part of Scandinavia, but neither is Iceland nor Greenland. However Finland is very much a part of the Nordic Countries (and so is Iceland and Greenland, et al.). And people sort of use the terms “Scandinavia” and “The Nordic Countries” interchangeably, as the distinction isn’t that important[1]. So for for all intents and purposes (barring being pedantic) Finland is a part of Scandinavia.

[1]: I’m not an expert on this, but I doubt there are many inter-Scandinavian cooperation happening any more that excludes the other Nordic Countries.