Henry Winkler is among those claiming the deadly blazes ravaging L.A. are definitely arson ... but unless he has some proof, fire officials tell TMZ The Fonz needs to chill.

Following yet another blaze that started Wednesday night -- this time in the Hollywood Hills -- HW exclaimed on X ... "THERE IS an ARSONIST here in LA . May you be beaten you unrecognizable !!! The pain you have caused !!!"

Here's the issue ... there's no way to know at this moment exactly where and how the series of fires started ... but an investigation is underway, an L.A. Fire official tells TMZ.

Fires have torn through the Pacific Palisades, Altadena, the Hollywood Hills, Acton, and Sylmar, with embers swirling across the city sparking smaller blazes. At least 5 people have died as a result.

The L.A. department team investigating each fire is the arson squad -- but that's the team that investigates the cause of every major fire ... it doesn't mean arson is suspected at this point, we're told.

Now, if Henry or anyone else has actual evidence there is an arsonist involved in any or all fires -- that is something authorities are very much interested in knowing!