Neoplasticism or neo-plasticism, originating from the Dutch Nieuwe Beelding, is an avant-garde art theory proposed by Piet Mondrian in 1917 and initially employed by the De Stijl art movement. The most notable proponents of this theory were Mondrian and another Dutch artist, Theo van Doesburg. Neoplasticism advocated for a purified abstract art, by applying a set of elementary art principles. Thus, a painting that adhered to neoplastic art theory would typically consist of a balanced composition of simple geometric shapes, right-angled relationships and primary colors.

The term 'plastic arts' comes from the Greek word plastikos, which means "to mold or shape". This word perfectly describes the nature of plastic arts, which involve the use of materials that can be molded or shaped.

Mondrian, Van der Leck and Van Doesburg first set out the philosophical basis for the art theory known originally as "Nieuwe Beelding", but known today as "Neoplasticism", in a new art journal named De Stijl [The Style]. The term appears in an editorial by Van Doesburg in the first issue of the journal and in the first of a series of articles by Mondrian entitled De Nieuwe Beelding in de schilderkunst. The expression "nieuwe beelding" is believed to derive from the work of Mathieu Schoenmaekers, who used the term in his 1915 book Het Nieuwe Wereldbeeld,; copies of books by Schoenmaekers were found in Mondrian's library.

Introducing their translation of Mondrian's publications, Holtzman and James wrote:

The Dutch verb beelden and substantive beelding signify form-giving, creation, and by extension image – as do gestalten and Gestaltung in German, where Neo-Plastic[ism] is translated as Die neue Gestaltung. The English plastic and the French plastique stem from the Greek plassein, [meaning] to mold or to form, but do not quite encompass the creative and structural signification of beelding.

Some authors have translated nieuwe beelding as new art.

The term néo-plasticisme [neo-plasticism] first appeared in Mondrian's Le Néo-plasticisme: Principe Général de l'Equivalence Plastique, [Neo-plasticism: the general principle of plastic equivalence]. Mondrian described the essay as a "condensed adaptation of the ideas in his Trialogue". The book was translated into French with the help of Mondrian's old friend, Dr Rinus Ritsema van Eck. In the 1925 German edition – the fifth in the Bauhaus Bauhausbücher series (translated by Rudolf F. Hartogh) – the term néo-plasticisme is translated as Neue Gestaltung [New Design].

Between 1935 and 1936, Mondrian wrote an essay in French, translated into English with the help of Winifred Nicholson and published in the book Circle: International Survey of Constructive Art as "Plastic Art and Pure Plastic Art (Figurative Art and Non-Figurative Art)". After moving to the United States, Mondrian wrote several articles in English with the help of Harry Holtzman and Charmion von Wiegand, in which he maintained the use of the term 'plastic'.