The plaques commemorate loved ones who have been cremated at Overdale and are displayed on stones at the site, which is adjacent to Heaton Cemetery.
The crematorium is owned and operated by Bolton Council, and the matter has been passed to Greater Manchester Police. It follows a similar incident earlier in the year.
Council officers believe the thieves may try to sell the plaques to scrap metal merchants.
But tighter rules mean that this is now more difficult, as anyone supplying scrap to a merchant must provide photo ID and a bank account, as payment in cash is illegal.
In addition, the plaques are made of gunmetal, which is worth considerably less than brass and other metals.
Bolton Council’s Executive Member for Environment, Cllr Richard Silvester, said:
“This kind of cruel theft beggars belief and I want to extend my sincere sympathy to anyone affected.
“The council will be working with the police to identify those responsible.
“I urge anyone who notices that the plaque for their loved one is missing to get in touch with the crematorium and we will help to get it replaced.”
Anyone affected is asked to contact Overdale Crematorium on 01204 334499.
