The show, which stars Bradley Walsh, Jamie Bamber, Harriet Walter and Freema Agyeman, is based on the successful American Law & Order format.
The third series of the show premiered last month and attracted guest stars including Rupert Graves, Deborah Findlay and Kevin Doyle.
Juliet Stevenson, Tobias Menzies and Nicola Walker have already signed up to appear in the next series.
ITV's director of drama Laura Mackie said: "Law & Order: UK is continuing to deliver audiences with its top-flight acting and 'ripped from the headlines' storylines that captivate and entertain. With the commissioning of 13 new episodes we are proving ITV's commitment to a series which has the potential to run and run, much like its US counterpart."
The fourth series of Law & Order: UK is expected to air in the new year.
ATV Network has some additional information about the renewal. Richard Stokes will continue as producer on the next series (which will likely be split into Series 5 and Series 6). Emilia di Girolamo is co-producer and lead writer (personal sidenote: YAY!) and other writers will include Debbie O'Malley, Nick Hicks-Beach, Roy Williams and Richard Stokes.