The 21-year-old confirmed Arthur as her boyfriend while sharing a pic of him giving her a piggyback ride -- all for a photo diary of her Stanford University graduation for Nylon. She wrote, "My boyfriend, Arthur, giving me a lift post-ceremony."

Later in the evening, Phoebe shared another candid pic of her cuddling up with Arthur, gushing about how he "cleans up nicely."

Phoebe's romance with Arthur wasn't exactly a secret sauce -- they ignited dating rumors back in October 2023 ... after sharing an IG photo from a trip to Paris.

The pair also attended the "Feud: Capote vs. The Swans" premiere earlier this year. However, in a March 2024 Bustle interview, Phoebe declined to comment on their relationship status.

Phoebe is the youngest of Bill's brood -- one of 3 he shares with ex-wife Melinda Gates. Meanwhile, Arthur is the eldest son of Mary McCartney and her ex-husband, TV producer Alistair Donald.