The actress – who played Bex Reynolds in Casualty – will make her debut on the show in February 2010.
Sarah said: “I’m over the moon to be joining The Bill, especially at this exciting time for the show.
“Thanks to a friend who is a Sergeant in the Metropolitan Police, I have been lucky enough to spend time shadowing Pc’s out and about around London as research for my new role.”
She added: “I can’t wait to get behind the wheel and join the team at Sun Hill.”
The Bill producer Tim Key explained: “Sarah’s a great actor and we’re delighted that she’s joining the show. She arrives at a particularly exciting time for the uniform officers of Sun Hill, who are about to be tested to the limit.
“Kirsty Knight is a lively addition to the team, with an irreverent sense of humour and a love of the adrenaline-fuelled side of the job.
“But she’s going to be flung in at the deep end and will quickly learn that life at Sun Hill is full of surprises, challenges and tough decisions.”