Millie Bobby Brown is spreading Christmas cheer with her new line of underwear ... giving fans an early gift by dressing up as a sexy Elf on the Shelf.

This outfit probably won't keep Millie warm at the North Pole ... after all, it's just a skin-baring crop top and a pair of booty shorts with a thick black belt and a long Santa hat -- though it'll certainly get her fans feelin' a little hot under the turtleneck.

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Millie said she's not an Elf on the Shelf ... but, instead, a "Mill on a sill" -- ready to report back to Santa on any naughtiness she spies.

MBB also shared a video where she describes the full 'fit ... ending the clip with a cheerful "ho, ho, ho" and flirty laugh

Of course, Millie's already giving "Stranger Things" fans a huge Christmas present ... because the second half of the series' final season will come out on Christmas Day.

The show might be a better fit for Halloween, to be fair ... but, after the huge hiatus between Season 4 and Season 5, no one's kicking up too much of a fuss.

Deck the hall with boughs of holly ... and, your own Mill on a Sill, if you can somehow find one!