Pieter Casteels III was a Flemish painter and engraver mainly known for his flower pieces, game pieces and bird scenes. He spent a significant portion of his life in England where he had a varied career as a still life painter, printmaker and textile designer.

Pieter Casteels III was born in Antwerp as the son of Elisabeth Bosschaert and Pieter Casteels II, a painter of landscapes and history paintings. He trained with his father. In 1708 he left with his brother-in-law Peter Tillemans to England to work for a picture dealer named Turner for whom they made copies of Old Master paintings. Casteels became an active participant in London's artistic community, subscribing to the Kneller Academy of Painting and Drawing in 1711 and becoming a member of the Rose and Crown Club. He returned briefly to Antwerp in 1712 where he became a member of the local Guild of Saint Luke in the same year.

Casteels settled permanently in England around 1717. He developed a successful practice as a painter of flowers and exotic birds that chiefly served a decorative purpose as overdoors and chimney-pieces. He worked simultaneously as an art dealer and imported paintings from Europe. His customers included James Stanley, 10th Earl of Derby who bought imported art as well as original work of Casteels.

In 1726 Casteels launched a subscription for a set of 12 prints of birds, which he had etched after his own designs. The success of this project encouraged him to work on two further publications: the Twelve Months of Flowers and the Twelve Months of Fruit.

Casteels advertised the usefulness of the illustrations in these publications as patterns for workers in luxury industries. Casteels was thus able to demonstrate his potential as a textile designer. In May 1735 he retired from painting and spent his last fourteen years working for a calico manufacturer as a residential artist, first at Martin Abbey near Tooting, Surrey, and later, briefly, in Richmond, London.

He died on 16 May 1749 in Richmond after a long illness.