Smoking in enclosed public places has been banned from early on Sunday.
Police say they will enforce the smoking ban
The ban came in at 0400 BST and extends to bars, restaurants, clubs and offices in the island.
Guernsey's environmental health department will make routine checks to ensure it is being complied with and also to iron out teething problems.
Staff have been trying to ensure people are up to date with the law's details and Guernsey Police said they were ready to enforce it.