The decision was taken “to alleviate the suffering” of residents in the Palestinian enclave, Sisi said on Facebook on Thursday.
It is usually only open for a few dozen days a year.
A total of 114 Gazans have been killed since 30 March, in the run-up to the opening on Monday of the US embassy in Jerusalem.
Egypt has relations with both Israel and Hamas, the rulers of Gaza , and Cairo plays a key role in easing tensions between the two sides and alleviating the pressure on the enclave’s 2 million residents.
The Hamas leader, Ismail Haniyeh, paid a brief visit to Egypt on Sunday, where he met the director of the country’s intelligence services, Abbas Kamel.