Charles Alexander Jencks (born 21 June 1939) is an American architectural theorist, landscape architect and designer. His books on the history and criticism of Modernism and Postmodernism are widely read in architectural circles. Jencks now lives in Scotland where he designs landscape sculpture and writes on cosmogenic art.

Born in Baltimore, Jencks first received his BA in English Literature at Harvard University in 1961, later gaining an MA in architecture from the Graduate School of Design in 1965. In the mid-sixties Jencks moved to Scotland where he lived with his late wife Maggie Keswick Jencks. He took his studies even further in 1970, receiving his PhD in Architectural History from University College, London.

The Garden of Cosmic Speculation, begun in 1988, was dedicated to Jencks' late wife Maggie Keswick. The garden has such a name because Jencks, Keswick, scientists, and their friends designed the garden based on natural and scientific processes. Jencks goal was to celebrate nature, but he also incorporated elements from the modern sciences into the design. The garden contains a species of plants that are pleasurable to the eye, as well as edible. With a ‘century of extraordinary discovery in biology’ like evolution and deoxyribonucleic acid also known as DNA, and cosmology, this has given birth to a new type of garden design [Cosmic]’. Preserving paths and the traditional beauty of the garden is still his concern, but Jencks enhances the cosmic landscape using new tools and artificial materials. Just as Japanese Zen gardens, Persian paradise gardens, the English and French Renaissance gardens were analogies of the universe, the design represents the cosmic and cultural evolution of the contemporary world. The garden is a microcosm - as one walks through the gardens they experience the universe in miniature. According to Jencks, gardens are also autobiographical because they reveal the happiest moments, the tragedies, and the truths of the owner and family.