Princess Hemvadi (Thai: พระเจ้าบรมวงศ์เธอ พระองค์เจ้าเหมวดี), informally known as Sadet Phra Ong Hem, was the youngest daughter of King Chulalongkorn (Rama V) and Hem Amatayakul (daughter of Phraya Thammasarnniti Wichitphakdi (Plub Amatayakul). She was born on 12 January 1892 (1893 according to the modern calendar). Originally named Princess Mandhanabhavadi, her father, King Chulalongkorn, renamed her Princess Hemvadi on 8 January 1898 (1899 according to the modern calendar) when she was six years old.

Princess Hemavadi resided at Ratchathat Palace, which is currently the site behind the Anantara Siam Bangkok Hotel on Ratchadamri Road, Bangkok.^{[citation needed]}

She was one of the few daughters of King Rama V who lived through the reign of King Bhumibol Adulyadej (Rama IX) and was among the last surviving members of her generation. The Princess died on 17 October 1972 from renal failure at the age of 80. Their Majesties King Bhumibol Adulyadej and Queen Sirikit presided over her royal cremation ceremony on 28 February 1973 at the Royal Crematorium, Wat Debsirindrawas Ratchaworawiharn.