John Douglas MacLachlan (July 30, 1906 – October 13, 1987) was a Canadian botanist and the first president of the University of Guelph.
Born on a farm near Burritts Rapids, Ontario, he received a Bachelor of Arts degree in chemistry and biology at Queen's University in 1931.
He received a Master of Arts degree in 1933 and Ph.D. in plant pathology in 1935 from Harvard University.
In 1939, he was appointed assistant professor of botany at the Ontario Agricultural College.
MacLachlan Building at the University of Guelph is named in his honour.