The magazine, owned by Wilmington Group, was originally planned to become an online-only title, but a compromise was decided on.
Press Gazette has been in weekly format for 43 years. In its new A4 format, the magazine will focus on interviews and features, while the website will cover breaking news.
Sam Bryant, production partner at Manson Group, said she was unsure as to how the volumes would be affected, but added: "I understand the managing director has been in consultation about the decision for a while."
Wilmington Group bought the troubled magazine in December 2006, following a period of co-ownership by former Daily Mirror editor Piers Morgan and PR executive Matthew Freud.
Press Gazette went into administration for a short period, before it was bought by Wilmington.
There has been no confirmation about what the changes mean for the title's seven journalists and five commercial staff.
Press Gazette print run reduced as it switches to monthly publication
Manson Group, the St Albans-based printer, will reduce its run of journalist's trade publication Press Gazette after the title switched its circulation from weekly to monthly.
