A soothsayer was someone who could foresee the future.

In an alternate timeline where all of history occurred at once, the Eleventh Doctor served as soothsayer to the Holy Roman Emperor, Winston Churchill. (TV: The Wedding of River Song)

According to one account, the Doctor was the soothsayer who told Julius Caesar to "Beware the Ides of March." (PROSE: Doctor Who and the Stones of Blood)

A Verron soothsayer once gave Sarah Jane Smith a puzzle box (TV: Whatever Happened to Sarah Jane?) and a warp star (TV: Journey's End) after Sarah Jane and her friends helped him return to his home planet. The soothsayer told her to give the puzzle box to the person she trusted most. She gave it to Maria Jackson. (TV: Whatever Happened to Sarah Jane?)

In Pompeii there were many soothsayers who had foreseen the arrival of the Tenth Doctor, though they had not foreseen the disaster that would happen shortly after the Doctor's arrival despite the fact that it was a fixed point in time. (TV: The Fires of Pompeii)

After hurtling through the time vortex to save Davros, Dalek Caan became a soothsayer for the new Dalek Empire. Travelling through the time vortex to the time-locked Time War to rescue Davros caused Dalek Caan to "see time", yet it also cost him his sanity. The Daleks, especially the Supreme Dalek, saw him as an abomination, but never dared to contradict Dalek Caan's prophecies that the Doctor was returning. (TV: The Stolen Earth/Journey's End)