The 77-year-old MP announced in February she would retire after more than 30 years in Parliament but said in September she had changed her mind.
Ms Clwyd faced three other contenders on an all-women shortlist.
Labour officials had said she would have to seek re-selection as the party had already started the process of replacing her.
The other candidates were PCS union official Katie Antippas, from Cardiff; Rhondda Cynon Taf councillor Sue Pickering; and scientist Aysha Raza, a Labour councillor in Ealing, west London, originally from Aberdare.
Ms Clwyd has represented the Cynon Valley seat in south Wales since winning a by-election there in 1984.
She said in September that local voters had asked her to reconsider her retirement, adding that she then received a "very enthusiastic" reaction to her change of heart.
It followed a row over Labour's decision to use an all-women shortlist to select her successor, which had been strongly opposed by local party officials.
MP Ann Clwyd has won the selection process to defend her Cynon Valley seat at the 2015 General Election.