Harris is now on TikTok ... joining the platform Thursday to keep building on her momentum with Democrats, which has been obvious since President Biden dropped out of the race Sunday and endorsed her for prez.

The current veep's first TikTok post was brief and to the point, explaining why she started an official account -- "Well, I've heard that recently I've been on the For You Page, so I thought I'd get on here myself."

As of this post, it's only been a little over an hour since she joined, and she's already collected more than 300,000 followers ... with 349K likes.

Kamala's TikTok bio says ... "When we fight, we win."

As you know, she's barreling toward becoming the Democratic nominee for president and is picking up endorsements left and right from prominent Dems and celebs.

Now, she's making a very strategic play for the Gen Z vote ... a demographic that could be key to her presidential aspirations.

For his part, Donald Trump is already on TikTok -- he joined in June, but he knew he would be the Republican nominee long before Harris knew she was getting the nod for her party. Trump has 9.2 million followers and 23 million likes.