Johannes de Sacrobosco or Sacro Bosco (John of Holywood, c. 1195 - 1256) was an English scholar who taught at the University of Paris.

Although described as English, his birthplace is unknown because Sacrobosco is an otherwise unknown town or region. The theory that he was born in Halifax is now discredited because Halifax means 'holy hair', not 'holy wood'. He was educated at Oxford University. According to a seventeenth century account, he arrived at the University of Paris on June 5, 1221, but whether as an arts student or as a licentiate (one having a Master of Arts degree from another university and thus qualified to teach) is unclear. In due course, he became a Professor of Mathematics at the University of Paris. About 1230, his most well known work, Tractatus de Sphaera, was published. In this book, Sacrobosco discussed the Earth and its place in the Universe. It was required reading by students in all Western European universities for the next four centuries. Its description of the Earth as a sphere and its popularity exposes the nineteenth-century opinion that medieval scholars thought the Earth was flat as a fabrication. Sacrobosco was a strong proponent of Arabic methods of mathematics, being one of the first Western Europeans to use Arabic numerals and sexagesimal notation.

What Sacrobosco may be most famous for is his criticism of the Julian calendar. In his book De Anni Ratione (1235), he maintained that the Julian calendar was ten days off and that some correction was needed. His plan to fix it was to skip one day every 288 years. In this book, he invented the false notion that Caesar Augustus took a day from February to give to August (see Julian calendar).

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• Johannes de Sacrobosco