Is there a sixth book in the Noughts and Crosses series by Malorie Blackman?

No, there isn't a sixth book in the Noughts and Crosses series by Malorie Blackman. The series consists of five books:

1. Noughts & Crosses (2001)

2. The Knife Man (2002)

3. Checkmate (2003)

4. Crossfire (2004)

5. Double Cross (2005)

While there are no further installments in the main series, Blackman has written a prequel novella, The Time They Stood Still, which delves into the history of the Noughts and Crosses world.