Alastair Harvey MacLennan, AO, MB ChB, MD, FRCOG, FRANZCOG (born 28 March 1945) is a Scottish-Australian physician, professor of obstetrics and gynaecology, medical researcher, and a community health advocate.

He studied and practised medicine in Glasgow, Chicago, and Oxford before moving to Australia in 1977 to take up a position at the University of Adelaide, where he went on to become the Professor and Head of the Discipline of Obstetrics and Gynaecology in 2006.

At age 16, he began studying at the University of Glasgow Medical School, mentored by Ian Donald, who pioneered obstetric ultrasound.

He graduated in 1968 with a MB ChB, choosing to specialise in obstetrics and gynaecology because it incorporates being both a physician and a surgeon, with a variety of sub-specialities and research opportunities.

[1] In 1970, MacLennan started as a Research Fellow at the University of Illinois at Chicago, describing the menstrual cycle and placentogenesis of the baboon.

[8] After retiring from his full-time academic position, he remains on the emeritus staff of the Robinson Institute, where his research team challenged the myth that cerebral palsy is usually due to lack of oxygen at birth.

[11][12] In May 2018 they announced the results of a large-scale genetic study which identified disruptions in RNA signalling and inflammatory pathways that are common to children with cerebral palsy and autism.

[8] His service to professional organizations include chairing the International Expert Task Forces and Consensus Statements on Cerebral Palsy in 1995, 1999 and 2003; founding member and past president of the Perinatal Society of Australia and New Zealand; chief Australian Investigator of the Women's International Study on Long Duration Oestrogen after Menopause ("WISDOM"); founding member and past president of the Australasian Menopause Society.

His initial focus was on the damage caused by chiropractic treatment to new-born babies, and the abuse of non-prescription hormone replacement substitutes.

[18] He expressed particular concern about the lack of understanding in the general public: his longitudinal study in 2004 observed that "the public is often unaware that CAMs are not tested by the Therapeutic Goods Administration for efficacy or safety"[19] – on the contrary, his studies found that most Australian consumers believe that alternative preparations have been tested to determine whether they are safe or effective, when they have not.

She became a Clinical Senior Lecturer at the University of Adelaide,[16] Chair of the Council of Affiliated Menopause Societies from 2005 until 2008,[2] and was awarded a Medal of the Order of Australia (OAM) in 2012 for her work in women's reproductive health.