Two more people including a 12-year-old were badly wounded.
Friday's violence came a day after jihadists targeted security forces with rockets and a car bomb in North Sinai province in simultaneous attacks claimed by an affiliate of the Islamic State group.
Security officials said the bodies of the 30 victims, most of them soldiers, had been flown to Cairo.
Sisi pulled out of an African Union summit in Ethiopia and flew home "to monitor the situation", his office said.
It was the deadliest wave of attacks since October when 30 soldiers were killed and scores wounded in simultaneous assaults.
"The army and police will intensify their raids against terrorist and extremist elements in Sinai and across the country," a military statement said.
The fresh bloodshed came despite new security measures.
Jihadists have regularly targeted security forces in the Sinai since Islamist president Mohamed Morsi was ousted by then army chief Sisi in 2013.
- Militant 'retaliation' -
The militants say the attacks are in retaliation for a government crackdown on Morsi supporters in which hundreds have been killed, thousands jailed and dozens sentenced to death.
US State Department spokeswoman Jen Psaki condemned Thursday's attacks and said Washington "remains steadfast in its support of the Egyptian government's efforts to combat the threat of terrorism".
Late last year, Washington delivered 10 Apache helicopters to Egypt, which has poured troops and armour into the peninsula, for joint counter-terrorism operations in the Sinai.
On Friday, Iran stressed the need for regional cooperation "to combat the terrorist menace in Egypt".
The main focus of Thursday's attacks was El-Arish, the provincial capital, where militants fired rockets at a police headquarters, a military base and a residential complex for security forces, officials said.
This was followed by a suicide car bombing and an attack on a military checkpoint south of El-Arish.
Separately an army officer was killed when a rocket struck a checkpoint in Rafah, on the border with the Gaza Strip.
Officials said at least 62 people were wounded in the attacks.
The Islamic State group's Egyptian affiliate, Ansar Beit al-Maqdis, claimed the assaults, saying on Twitter it had "executed extensive, simultaneous attacks in the cities of El-Arish, Sheikh Zuweid and Rafah".
The organisation in November pledged allegiance to IS, which has captured large chunks of territory in Syria and Iraq.
Elsewhere, a policeman was killed by a bombing in the canal city of Suez, and a suspected militant mistakenly blew himself up in Port Said while planting a device.
- Security measures -
The October attack, also near El-Arish, prompted the authorities to build a buffer zone along the Gaza border to prevent militants infiltrating from the Palestinian enclave.
They have also imposed a state of emergency and night-time curfew in parts of North Sinai.
"These measures are just random retaliation that creates more terrorism, and what is worse is that it could make residents sympathise" with the extremists, said Ahmed Abdel Rabo, political science professor at Cairo University.
The military said Thursday's attack was the result of "the failure of Muslim Brotherhood... in spreading chaos on the fourth anniversary of the 25 January revolution".
On January 25, 2011, millions of Egyptians protested against president Hosni Mubarak, eventually forcing him to step down.
As Egypt marked the anniversary Sunday, clashes between protesters and police left 20 people dead, mostly in Cairo.
Since Mubarak's ouster Egypt has been rocked by political and economic turmoil.
Jihadists have killed scores of police and soldiers, mostly in the Sinai. The authorities have blamed these attacks on Mubarak successor Morsi's Muslim Brotherhood, which denies the charges.
Ansar Beit al-Maqdis has claimed most of the attacks, including the deadly October assault.