Brenda Lee finally has another #1 hit on her hands -- and it's the holiday classic she cranked out more than 6 decades ago ... which finally topped a major chart all these years later.

The songstress' famous 'Rockin' Around the Christmas Tree' topped Billboard's Hot 100 list Monday, just in time for the holidays. If you can believe it, this song has never reached No. 1 before this -- and it first got released waaaay back in 1958.

Now, 'RATCT' is the most popular song this week ... which is a huge milestone obviously. We should also note, the benchmark reached today has been a long time coming -- especially recently.

As with many things, BL's famed song grew in popularity as it got older ... and it's been hitting especially hard these past few years. Starting in 2019, 'Rockin' Around' began peaking on the Hot 100 at #2, and it stayed that way (around this time of year) for 3 years straight.

At long last ... the song has broken through and become #1, beating out Mariah Carey's beast of a Christmas song -- plus all the other hit singles out right now. BTW, this is Brenda's 3rd #1 single ... she hit no. 1 twice in 1960 with a couple ditties from back then.

Funny enough, Brenda is still singing this beloved classic even today ... including on airplanes.