Donald Olding Hebb (July 22, 1904-August 20, 1985) was a Canadian psychologist who was influentian in the area of neuropsychology, where he sought to understand how the function of neurons contributed to psychological processes such as learning.

His most famous work involved the rule of covariation of synaptic weight between two neurons, a process now known as Hebbian learning or more generally, Hebbian theory.

He spent much of his career as a lecturer of psychology at McGill University in Canada.

He came up with the A/S ratio, a value that measures the brain complexity of an organism.