In molecular biology Type I topoisomerases are enzymes that cut one of the two strands of double-stranded DNA, relax the strand, and reanneal the strand. They are further subdivided into two structurally and mechanistically distinct topoisomerases: type IA and type IB.

• Type IA topoisomerases change the linking number of a circular DNA strand by units of strictly 1.
• Type IB topoisomerases change the linking number by multiples of 1 (n).

Historically, type IA topoisomerases are referred to as prokaryotic topo I, while type IB topoisomerases are referred to as eukaryotic topoisomerase. This distinction, however, no longer applies as type IA and type IB topoisomerases exist in all domains of life.