Tadeáš Hájek z Hájku (Czech pronunciation: [ˈtadeaːʃ ˈɦaːjɛk ˈzɦaːjku]) (1 December 1525 in Prague – 1 September 1600 in Prague), also known as Tadeáš Hájek of Hájek, Thaddaeus Hagecius ab Hayek or Thaddeus Nemicus, was a Czech naturalist, personal physician of the Holy Roman Emperor Rudolph II and an astronomer in the Kingdom of Bohemia.

He published the Aphorismi Metoposcopici in 1561, dealing with divination and diagnosis by interpreting lines on the forehead.

In 1564 he received the Emperor's privilege stating that no astrological prognostication could be printed in Prague before he had seen and approved it.

His voluminous writings in Latin were mostly concerned with astronomy and many regarded him as the greatest astronomer of his time.

Throughout his life he also published numerous astrological prognostics in Czech and that is why he was until recently viewed as an "occultist" rather than a great scientist.