Noth, who left the original "Law & Order" 10 years ago, will become a regular on the spinoff beginning next season, series spokeswoman Pam Ruben Golum said Tuesday.
He will split duties with current series lead Vincent D'Onofrio (search), who's continuing as police Detective Robert Goren. Each actor will appear in 11 episodes for the fifth season. The NBC series airs at 9 p.m. EST Sunday.
Noth's casting will ease the work load for D'Onofrio, who was hospitalized briefly last year for exhaustion.
"The hardest job in show business is being a single lead on an hour drama series," said "Law & Order" creator and executive producer Dick Wolf (search). "Vincent has done an unbelievable job for the last four seasons, but after 3 1/2 years, the grueling pace finally took its toll."
He called Noth's addition to the show an "ideal solution."
Noth, who was Sarah Jessica Parker's love interest, Mr. Big, on HBO's "Sex and the City," will again play police Detective Mike Logan, his character from 1990-95 on "Law & Order."