What is the synonym of gullible?

Here are some synonyms for "gullible," ranging from formal to informal:

Formal:

* Credulous

* Naive

* Trusting

* Unsuspecting

* Dupable

Informal:

* Easily fooled

* A sucker

* A patsy

* A sap

* A mark

The best synonym to use will depend on the context and the desired tone.