How do you say I have seen this before in french?

There are a few ways to say "I have seen this before" in French, depending on the context and the nuance you want to convey:

Formal:

* J'ai déjà vu ça. (Most direct translation)

* J'ai déjà vu cela. (More formal and slightly less common)

Informal:

* Je connais ça. (I know this, implying familiarity)

* J'ai déjà vu ça quelque part. (I've already seen this somewhere)

* Ça me dit quelque chose. (This rings a bell)

More specific:

* Je l'ai déjà vu. (I've already seen it, focusing on the object)

* J'ai déjà vécu ça. (I've already experienced this)

* Je me souviens d'avoir déjà vu ça. (I remember having already seen this)

The best way to say it depends on the situation and the level of formality you need.