The new legislation, adopted last week by parliament after heated debates, provides expanded scope for the MIT agency to tap private phone conversations and collect intelligence related to terrorism and international crimes.
It also offers spy agents greater immunity from prosecution and provides for prison terms of up to 10 years for journalists and others who publish leaked information.
The law was signed by Gul late Friday and came into force after being published in the Official Gazette on Saturday.
It is seen as the latest ammunition being deployed by Prime Minister Recep Tayyip Erdogan after leaked telephone conversations implicating him in a widening corruption scandal and revealing high-level security talks on Syria became public.
Erdogan’s Justice and Development Party (AKP) argues the law will make the agency more efficient.
Erdogan, in power for 11 years, has accused former ally Fethullah Gulen, a US-based Muslim cleric, and his loyalists in the police and the judiciary, of being behind the corruption probe and the leaks.
The government has reacted by embarking on a mass purge of police and prosecutors and launching an Internet crackdown that saw Twitter, used to spread the leaks, banned for two weeks.
Despite the corruption scandal and the highly criticised measures, the AKP won a resounding victory in March 30 local elections.