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Professor Alexander Thom (1894–1985) was a Scottish engineer most famous for his theory of the Megalithic yard.

A graduate of the University of Glasgow, he returned there as a lecturer from 1922–1939. Thom later became a professor of engineering at the University of Oxford when he became interested in the methods used by prehistoric peoples in building megalithic monuments especially the stone circles of the British Isles. He travelled in the company of his son Archie, measuring prehistoric sites and analysing the data created.

In 1955, he published A statistical examination of megalithic sites in Britain in which he first suggested the megalithic yard as a standardised prehistoric measurement. He also attempted to classify stone circles into different morphological types.

Thom went on to identify numerous solar orientations at stone circles which led him to argue for a prehistoric calendar of 8 'months' divided by midsummer, midwinter and the two equinoxes and then subdivided by early versions of the modern Christian festivals of Whitsun, Lammas, Martinmas and Candlemas (see Scottish Quarter Days). He explored these topics further in his later books, Megalithic sites in Britain (Oxford, 1967), Megalithic lunar observatories (Oxford, 1971) and Megalithic Remains in Britain and Brittany (Oxford, 1978), the last written with his son Archie after they carried out a detailed survey of the Carnac stones from 1970 to 1974.

Thom's ideas met with resistance from the archaeological community but were welcomed amongst elements of 1960s counter-culture. Along with Gerald Hawkins' new interpretation of Stonehenge as an astronomical 'computer', Thom's theories were adopted by numerous believers in the lost wisdom of the ancients and became commonly associated with pseudoscience which saddened him greatly.

That stone circles had some astronomical significance to Neolithic peoples is nowadays rarely disputed and Thom has been partly vindicated although his megalithic yard is still a contentious topic.

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