Thompson, 60, who announced earlier this year that he will leave the Senate when his term expires in January, will become the first member of Congress to appear as a TV show regular while still in office.
The 13th season of "Law & Order," currently the longest- running show on TV, premieres Oct. 2, with Thompson joining the cast as the newly elected New York City district attorney.
Thompson's character replaces interim D.A. Nora Lewin, played for two seasons by Dianne Wiest, who will not return to the show. She took over for Steven Hill, who had portrayed D.A. Adam Schiff since the series debut in 1990 and was the last original cast member to leave the series.
The election of Thompson's character is "definitely a reaction to 9/11," said Michael Chernuchin, an executive producer of the series. "His political leanings are a little more to the right than former D.A.s on the show. He is a 'strict constructionist.' That is, for him, the Constitution is what it says and it is nothing more."
Thompson also will appear occasionally on two spin-off series, "Law & Order: Special Victims Unit" and "Law & Order: Criminal Intent," according to Universal Television, a unit of Vivendi Universal .
The ripped-from-the-headlines drama about New York City police detectives and prosecutors ranked last season as the fifth-highest rated series on U.S. television. The show, from veteran producer Dick Wolf, has garnered a record 11 straight Emmy nominations as best drama series, winning the coveted award in 1997.
Thompson, who once said life in Washington made him "long for the sincerity and realism of Hollywood," announced in March he would not seek re-election this year because he "simply did not have the heart for another six-year term."
The Tennessee Republican, who previously served as a federal prosecutor and Senate Watergate counsel, won a special election to a two-year Senate term in 1994. He won a full six-year term in 1996.
Before his Senate career, Thompson appeared in more than a dozen Hollywood films, including "In the Line of Fire," "Die Hard 2" and "The Hunt for Red October."
Several lawmakers on Capitol Hill found fame on television before serving in Congress, including the late Sonny Bono, Fred Grandy, who played Gopher on "The Love Boat," and Ben Jones, who was Cooter on "The Dukes of Hazzard." And many incumbent politicians have made guest appearances on TV shows over the years.
By Steve Gorman