The soldiers were killed when suspected militants armed with rocket-propelled grenades struck two buses carrying security forces Monday in the city of Rafah, on the border between Egypt and Gaza, Nile TV reported.
Interim President Adly Mansour declared three days of national mourning following the attack, Nile TV said.
The Sinai Peninsula is a lawless area that was the site of frequent attacks even before Egypt's latest round of turmoil. In May, for example, seven Egyptian solders were kidnapped and held for six days there, a spokesman for Egypt's armed forces said.
But the attack adds to the persistent tension across the country since the military ousted the democratically elected Morsy in a coup.
Over the past week, about 900 people -- security personnel as well as citizens -- have been killed.
Deaths occurred when the military used force to clear two pro-Morsy sit-in sites in Cairo on Wednesday and violence raged after pro-Morsy supporters staged demonstrations Friday.