Katherine Helmond, a versatile actress who was especially known for her roles on the long-running television series “Soap” and “Who’s the Boss?,” died on Feb. 23 at her home in Los Angeles. She was 89.

The APA Agency, which represented her and announced her death on Friday, said the cause was complications of Alzheimer’s disease.

On “Soap,” ABC’s prime-time parody of daytime soap operas, which ran from 1977 to 1981, Ms. Helmond played Jessica Tate, a lovable aristocrat who was one of the show’s main characters. Some viewers, as is often the case, wrongly assumed that the character was her.

“After I did ‘Soap,’ ” she told The Boston Globe in 1998, “people thought that the casting director had gone to a dinner party or something and found this dizzy lady and brought her to Los Angeles. People are surprised to find out that I’m from a very working-class Irish Catholic family and that I didn’t come from a lot of money and go to a nice finishing school and live in a grand house in Connecticut.”