Tougher restrictions such as the closure of establishments that serve alcohol and major commercial facilities will be in place from Sunday through May 11.
The emergency declaration, covering roughly a quarter of Japan's population of 126 million and about a third of its economy, will come with just three months to go until the Tokyo Olympics begin.
Suga is set to finalize the decision at a COVID-19 task force meeting in the evening.
Although the state of emergency will not entail a hard lockdown like some other countries have imposed, spectators will effectively be banned from large events while train and bus operators will be asked to end operations earlier on weekdays, with fewer running on weekends and holidays.
Major commercial facilities including department stores and shopping malls will be asked to shut, with the exception of floor space to sell daily necessities.