Antonina Jadwiga Siemaszko (born July 14, 1970) is an American actress, best known for her film roles in Little Noises (1991), The Saint of Fort Washington (1993), and for her role as Eleanor Bartlet in The West Wing (2001–2006).

Siemaszko was born in Chicago. Her father, Konstanty Siemaszko, was a Polish-born Roman Catholic, and a survivor of the Polish Underground and the Sachsenhausen concentration camp. He escaped to the United Kingdom where he met Nina's mother, Collette McAllister. One of her brothers is actor Casey Siemaszko. Her other brother, Corky, is a writer for the Daily News and a reporter at NBC News. She attended The Theatre School at DePaul University. She played an undercover cop in Reservoir Dogs featured in a scene that wound up on the cutting room floor ensuring that the work became a rare example of a film containing no dialogue delivered by an actress. It was released on DVD. She played Mia Farrow in the CBS miniseries Sinatra. She played the leading role in Wild Orchid II: Two Shades of Blue.