Harriett Low Hillard (May 18, 1809 – 1877) was an American woman of letters and diarist. From 1829 to 1833 she lived in the Portuguese colony of Macau on the South China coast and she and her sickly aunt became the first American women to go to China. During her stay from 1829 to 1833, she wrote a journal in the form of letters to her older sister Molly (Mary Ann, 1808–1851), and became acquainted with many of the influential individuals in the colony. After her return to the United States, she married and moved to London, returning to New York with her husband and five daughters in 1848. Her journal is now part of the Low-Mills collection in the Library of Congress.

She was born Harriet Low, the second of twelve children of Seth and Mary Porter Low, in Salem, Massachusetts. Her father was a well-to-do merchant and owner of a successful shipping business among the ports of Salem, New York, London, and Canton (modern-day Guangzhou). A leading citizen of Brooklyn, Seth Low was one of the founders of the Unitarian church in that city. As one of four daughters in a large family, Harriet engaged in many household tasks, including sewing and mending.

In 1829 her uncle, American trader William Henry Low, and his wife Abigail Knapp Low (1795–1834), prepared to move to China for a five-year stay. While William Henry Low would be managing business interests for Russell & Co. in Canton, which was off-limits to women, his wife would be staying in Macau. They asked Harriet to accompany them and provide companionship for her aunt. The party boarded the Sumatra for a four-month voyage across the Atlantic and Indian Oceans, which included a three-week stopover in Manila. Harriet arrived in Macau on 29 September 1829, and took up residence at 2, Pátio da Sé, at the top of Calçada de S. João. She soon became acquainted with many of the well-known residents of Macau, including the painter George Chinnery, who painted her portrait; the Hong Merchant Howqua; and the surgeon Thomas Richardson Colledge. Through her uncle's connections, she also became familiar with all the employees of the East India Company along with other prosperous British merchants in the city. As the only unmarried young woman in the colony, she was invited to many "fancy balls, dances, teas and dinners".

Low had a strong desire to visit Canton, the only foreign trading enclave permitted in China at the time. However, under the regulations of the Thirteen Factory System, women were strictly forbidden from entering. Low and her aunt dressed up like boys, sailed to Canton, and went straight to the American Factory. When the Chinese discovered the women's true identities, they threatened to stop all trade in Canton forthwith, forcing Low and her aunt to leave.

During her stay in Macau, Low became secretly engaged to William Wightman Wood, a young naturalist from Philadelphia who was a founder and editor of the Canton Register, one of the first English language newspapers in China. When she informed her uncle of the engagement, he objected to her marrying a "penniless adventurer" and forced her to break off the arrangement.

In 1833, Low posed for a portrait by George Chinnery. She wore a low-cut dress in the latest fashion from Calcutta, with the sleeves stuffed with down pillows.

In 1833, Harriet departed China with the aunt and uncle to return to Salem. Her uncle, who was seriously ill, died on the way back to the United States.

In 1836, she married John Hillard (1813–1859), a son of English parents who was born in Richmond, Virginia. The couple settled in London, where John was a partner in a large bank. They had three sons and five daughters. Only the girls survived; Mary Hillard Loines was active in the women's suffrage movement.