Foster has been a regular on 5 Live since 2005, including previous appearances on Weekend Breakfast as a reporter. She is currently the station's regional reporter in the North East of England.
After studying at Durham University, she joined BBC Radio Cleveland (now BBC Tees) in 2002 and later enjoyed a two-year stint at Radio 1's Newsbeat.
In her role as 5 Live's regional reporter, Foster has covered a range of stories including the hunt for Raoul Moat and the Cumbrian floods. She has also deputised for presenters on Weekend Breakfast and 5 Live Drive.
Commenting on her new role, Foster said: "I'm thrilled to be joining Phil as co-presenter on Weekend Breakfast; we've worked together for quite a while now and have got to know each other really well. This really is my perfect job as I have the chance to combine two things that I love, presenting and reporting, and I can't wait to get started."
Radio 5 Live's controller Adrian Van Klaveren added: "Anna has already proved that she's a great journalist and has a warm personality and I think she and Phil will make a great combination each Saturday and Sunday morning.
"Weekend Breakfast is an important programme for us - it's vital for us in reporting the main news stories of each weekend, setting the agenda for the day's sport and capturing the mood of the weekend."
In February, it was announced that Rachel Burden would replace the outgoing Shelagh Fogarty as co-presenter of 5 Live Breakfast from May. Fogarty, who has presented the weekday breakfast show with Campbell since January 2004, will replace Gabby Logan on the 12pm to 2pm slot.