Six-year-old Meir Israel Eshosh passed away after being forgotten inside a car in the Gaza border area, Israeli media reported.
Eshosh is the nephew of Likud MK Shlomo Karhi, and was a resident of a moshav near Netivot.
Likud leader Benjamin Netanyahu offered Karhi condolences on behalf of himself and the rest of the party.
"Such a heavy and sad disaster," Netanyahu tweeted. "Sarah and I send him and the whole family condolences from the bottom of our hearts."