The Appropriate Adult star will play Reverend Julie Nicholson, whose daughter Jenny died when a bomb exploded at Edgware Road Underground station.
A Song For Jenny, written by playwright Frank McGuinness, is described as "a moving and precise exploration" of Mrs Nicholson's response to the events of 2005 - which left 52 people dead and injured more than 700.
Watson said: "Although daunted by the task ahead, as a Londoner who was there on 7/7, I feel honoured to be part of the team asked to tell this compelling story. I hope we can do it justice."
The programme will be filmed this year and broadcast 10 years after the attack.
BBC One controller Charlotte Moore said: "Ten years on, A Song For Jenny will be a fitting tribute to those who lost their lives in the tragedy of the 7/7 London bombings.
"But it will also be a very personal story - exploring universal themes of grief, loss and forgiveness through one mother's struggle to reconcile her feelings with her faith."
Other new BBC One shows announced include From Darkness - a crime drama by the writer of Sugar Rush Katie Baxendale about a female former police officer dragged back into her old life when four bodies from some of her old cases turn up.