Doctor Who is to have a new companion when the drama returns - but there will still be room for his current co-star.
Agyeman's character, a trainee doctor, arrived in the show in March
Actress Freema Agyeman is to play Martha Jones in three episodes of spin-off show Torchwood. She said it was a chance to "expand" her character.
She will miss the first half of the fourth series of Doctor Who, but will then return to the BBC One programme.
The new companion will be revealed soon and will appear in the entire 13-week run, which begins filming this month.
Former Crossroads actress Agyeman, 28, impressed producers with a brief appearance in the minor role of alien expert Adeola.
They invited her to try for the part of David Tennant's sidekick when Billie Piper left at the end of the second series. The show is in its fourth series since its revival in 2005, having ended previously in 1989 after 26 years.
The third series of Doctor Who, which concluded in the UK on Saturday, had gained "outstanding reviews", according to Russell T Davies, its executive producer and head writer.
Martha Jones will spend more time with Torchwood's Captain Jack
"Freema has been a huge part of that success, gaining rave notices for her portrayal of Martha," he added.
"Now we are taking the character of Martha into brand new territory with a starring role in Torchwood."
In May, Agyeman told BBC News how overwhelming it felt to be constantly recognised in public.
"Everyone's calling my name so it feels surreal," she said, as she walked up the red carpet at the Bafta TV Awards.