He succeeds Sally Dewar who became managing director of wholesale and institutional markets in January this year.
Justham is currently head of market infrastructure at the FSA.
The regulator says as director of markets, Justham will have responsibility for the UK Listing Authority, market monitoring, the supervision of exchanges and the oversight of the clearing and settlement infrastructure.
Justham’s career at the FSA began in January 2007 after joining from JPMorgan, where he was managing director at JPMorgan Cazenove.
Dewar says Justham brings valuable experience to the FSA and is highly regarded.
She says: “I have enjoyed working with him since he joined us last year and look forward to working closely with him in his new role.”