"I don't feel that... it properly represents what's going on in the world. It's a very outdated group of countries," Mr Trump said on Saturday.
The G7 group, which the US hosts this year, includes Canada, France, Germany, Italy, Japan and the UK.
The president said Russia, South Korea, Australia and India should be invited.
Speaking to reporters on board the presidential plane Air Force One, Mr Trump said that he was delaying the summit - which was scheduled to take place later in June - until September.