Frank Thayer Merrill was an American artist and illustrator. He is best known for his drawings for the first illustrated edition of Louisa May Alcott's novel Little Women, published in 1880. Over a five-decade career, he illustrated a wide variety of works for adults and children.

Frank Thayer Merrill was born on December 14, 1848, to George William Merrill (1824–1879) and Sarah Rose Merrill (née Alden, 1822–1895) in Roxbury, Massachusetts. Both parents were natives of Westbrook, Maine. His father was descended from Nathaniel Merrill, who emigrated from England to Massachusetts in 1635. His mother was a direct descendant of John Alden, of the Mayflower crew.

He attended local public schools, graduating from either Roxbury High School or Boston Latin School — sources disagree. (The 1865 state census lists the 16-year-old Merrill's occupation as "clerk" rather than "student".) His mother "greatly encouraged Merrill's artistic development and from her much of his talent is said to have come". Merrill participated in the free drawing program at the Lowell Institute from 1864 to 1875, and entered the school of drawing and painting at the Boston Museum of Fine Arts in 1875.

In 1870, Merrill established a studio in Roxbury where he dedicated himself to a career as an illustrator as well as painting in watercolor and oil. He traveled to Europe in 1884, painting and sketching in France, Switzerland, Belgium, The Netherlands and England.

He drew on his observations of Parisian life along the Seine river to write a story titled Through the Heart of Paris. It appeared in the January 1886 issue of the children's magazine Wide Awake and was included in a collection of Wide Awake travel articles, Sights Worth Seeing by Those Who Saw Them, also published in 1886.

Merrill's first major commission was to illustrate a new edition of Little Women. Originally released in two volumes in 1868 and 1869, a single-volume revised edition was published in 1880 to capitalize on the book's popularity and to deter copyright violators. Merrill created over two hundred pen-and-ink drawings for the new edition.

Over the course of his career, Merrill created thousands of illustrations for a wide variety of fiction and non-fiction for children and adults. Some of his earliest work appears in a history titled Pioneers in the Settlement of America published in 1877. He went on to illustrate a number of other histories, primarily for children, sharing his deep interest in American history, particularly the colonial and revolutionary eras.

Merrill married Jessie S. Aldrich (1858–1936) of Boston on December 14, 1881, at the Walnut Avenue (now Eliot) Congregational Church in Roxbury. The couple lived in Roxbury until 1886 when they moved to a newly built house and studio on Tremlett St. in the Codman Square neighborhood of Dorchester. Merrill worked out of this studio for the rest of his life.

A life-long resident of Boston, Merrill was active in the community. He gave talks to local groups on illustration and historic costuming, drawing on his experience collecting and restoring American antiques. In 1886, a charity event, "Music of the Centuries", was staged for the New England Conservatory. Merrill was credited for work on two tableaux: as artist for "Court of Charlemagne (A.D. 800)" and as artist and manager for "Cavaliers and Roundheads (A.D. 1645)". A Republican, he served as an inspector of elections for his city precinct. He was elected vice-president of the Corporation of Mount Pleasant Home for Aged Men and Women in 1908. Mrs. Merrill was also on the executive committee.

On December 14, 1931, Jessie and Frank Merrill celebrated their fiftieth wedding anniversary (which was also Frank's birthday) at the home where they had resided for 45 years, in the company of friends, and their three sons. Jessie died a few years later, followed by Frank within three months. Both were interred in the family plot at Forest Hills Cemetery in Jamaica Plain, Massachusetts.