The fatwa, or religious edict, issued by Suleiman al-Daya on Saturday night accords a religious legitimacy to the truce and could justify any act by Gaza's government to enforce it.
"Honouring the truce, which was sponsored by our Egyptian brethren, is the duty of each and every one of us. Violating it shall constitute a sin," the fatwa read.
The truce, agree on Wednesday, put an end to an eight-day Israeli offensive against Gaza militants who fired rockets into Israel.
The agreement remains fragile because details beyond the initial ceasefire have not yet been worked out.