The 'Bad Romance' star put fangs in her mouth and covered herself in Fake Blood as a kinky surprise for 'The Vampire Diaries' actor to mark the first time they have spent the romantic day together.
Explaining her costume, a source told The Sun newspaper: "She ordered it online about three weeks before the big day because he loves vampire-style sex sessions.
"They had a very laid-back evening at Taylor's house drinking lots of red wine and eating spaghetti."
Gaga also gave him a newly edited version of her music video 'You And I' - which he starred in, and is how he first met her - which had added footage of them both in it.
Gaga and Taylor are both fans of off-the-wall dates, and have previously romanced each other at a shooting range in Lancaster County, Pennsylvania.
A source said: "Gaga loves anything extreme. Watching a bunch of men unload guns would turn her on.
"Taylor and his pals like having her around as she's a laugh. She likes nothing more than heading to the pub for a few beers after."
Gaga has also been seen looking for properties in Pennsylvania, sparking rumours they are to move in together.