Nicholls will join the cast ahead of a sixth series, which is scheduled to air in the New Year.
The former Secret Diary of a Call Girl and Harley Street actor will play DS Sam Casey, who is described as a "headstrong young officer with a sense of justice".
He will be brought in as part of a storyline about the shooting of DS Matt Devlin (Jamie Bamber).
Bradley Walsh, Harriet Walter, Freema Agyeman, Peter Davison and Dominic Rowan will all return for the seven-part sixth run.
The fifth series of Law & Order: UK achieved a 5.6 million average audience for the broadcaster during its summer run.
Guest stars lined up for series six include Tamzin Outhwaite, Toby Stephens, Eva Pope, Luke Roberts and Tim McInnerny.
> Paul Nicholls leaves Waterloo Road after three days
> Paul Nicholls - Picture Special