Taylor Swift Definitely Bought a Mansion ... Next to Kennedy Crib

Taylor Swift is never, ever, ever ... gonna be more than a stone's throw from the Kennedys ... because TMZ has learned, Swifty DEFINITELY bought a mansion next door to their famous Cape Cod compound.

According to a local realtor ... Swift 100% will be moving across the street from Ethel Kennedy -- despite reports to the contrary. The realtor tells us the singer dropped around $4.5 million on the beachside mansion in Hyannis Port, MA.

As you know ... Taylor and 18-year-old Connor Kennedy -- grandson of the late Robert F. Kennedy and Ethel -- started dating a few months back.

Our sources tell us the deal will close in 3 weeks.

As for the crib -- we're talking 7 bedrooms, 6 baths, over 4,400 square feet, an acre of land and its own private beach.