Jessica Ellen Hayllar (1858–1940) was a British artist and painter.
Hayllar was born in London and was the eldest daughter of the nine children born to Ellen Phoebe Cavell (1827-1899) and her husband James Hayllar (1829-1920).
The family lived at Mecklenburgh Square in London and also rented a country house in Suffolk for several months each year before moving to a large house, Castle Priory, by the Thames at Wallingford, then in Berkshire. Hayllar and her four sisters attended a day school in Gower Street and all were given art lessons by their father, who was himself a well-regarded painter.
Jessica Hayllar became the most prolific artist among the Hayllar offspring, although her sister Edith also achieved some recognition.