The shell landed in the vicinity of Israel's western Negev region near the Karni checkpoint, causing no damage or injuries. No group has claimed responsibility for the attack.
The Israeli Army's press service denied media reports that an elderly resident of northern Gaza had been injured by shots fired from Israeli territory in response to the mortar. Israel says it did not retaliate for the mortar attack.
The incident was the first breach in the landmark Gaza ceasefire deal, agreed between the two sides through an Egyptian mediator in Gaza on June 17.
The six-month truce between Israel and Hamas-led militants, which came into force at 06:00 a.m. local time (03:00 GMT) on June 19, is limited to the Gaza Strip, and does not cover the West Bank, where Israeli troops killed two Palestinian militants in the city of Nablus early on Tuesday.
Palestinian security sources said a senior Islamic Jihad movement commander and a gunman belonging to the group were killed in an exchange of fire with Israeli forces. The IDF only confirmed the death of the Palestinian commander, who they said was planning an attack on Israel.