Nouns ending in -et are masculine.
The suffix often forms a diminutive or small object.
Is secret masculine or feminine?
lesecret
The rule behind it
99.2% of -et nouns are masculine
In context
Il y a un passage secret à gauche.
There's a secret path on the left.
Je lui ai conseillé de garder le secret.
I advised him to keep the secret.
Je lui ai promis de garder ça secret.
I promised him to keep it secret.