When used from the Bag on a wild Pokémon, it drastically increases its catch rate. The effect wears off if and when the wild Pokémon breaks out of a thrown Poké Ball.
The Golden Razz Berry was added to Pokémon GO on June 22, 2017. It can be used to make wild Pokémon significantly more likely to be caught, adding a ×2.5 multiplier to the catch rate. The Berry's effect lasts until the Pokémon breaks out of a thrown Poké Ball.
If a Golden Razz Berry is fed to a Gym defender, it will completely fill a Pokémon's motivation, regardless of how many Berries it has already eaten.