The CBS drama follows Carrie Wells (Poppy Montgomery), a detective who has perfect recall but cannot remember the details of her sister's murder.
Shutter Island actor Koteas will play a man who Carrie suspects may be the killer, reports TVLine.
His character Sam Rhodes is a former cop who left the force 27 years ago. When Rhodes is linked to the murder of a 14-year-old girl, Carrie notes the similarities between the incident and her sister's own death.
Koteas's past television work includes roles in House, CSI: NY and The Sopranos.
He is also known for his film credits, including 1990's Teenage Mutant Ninja Turtles, 1998's The Thin Red Line and 2008's The Curious Case of Benjamin Button.
Unforgettable's first season finale airs in the US on Tuesday, May 8. It is currently unclear if the show will be picked up for a second run, since it was not among CBS's initial batch of early renewals earlier this month.
In the UK, the series airs on Sky Living on Tuesdays at 9pm.
> Unforgettable Poppy Montgomery Q&A: 'Carrie is insanely reckless'
Watch a trailer for Unforgettable below: