Ultima Thule is a science fiction novella by American author Mack Reynolds, first published in 1962.

Three years after applying, Ronny is provisionally hired by United Planets and sent on a perplexing chase from planet to planet, trying to catch an interstellar saboteur. His quarry is subverting the peace and order UP was created to maintain.

Ronny Bronston has dreamed all his life of getting a United Planets job that would take him off-world. He finally gets the opportunity when he is given a provisional assignment with Bureau of Investigation, Section G. But will he be able to complete his assignment and find the elusive Tommy Paine?

Ultima Thule was first published in Analog Science Fact & Fiction March 1961.

Dallas McCord "Mack" Reynolds (1917-1983) was an American science fiction writer. His pen names included Dallas Ross, Mark Mallory, Clark Collins, Dallas Rose, Guy McCord, Maxine Reynolds, Bob Belmont, and Todd Harding.

His work is noteworthy for its focus on socioeconomic speculation, usually expressed in thought-provoking explorations of Utopian societies from a radical, sometime satiric, perspective. He was a considerably popular author from the 1950s to the 1970s, especially with readers of science fiction and fantasy magazines. 

Reynolds was the first author to write an original novel based upon the 1966-1969 NBC television series Star Trek, named Mission to Horatius (1968).