Diamanti cost the Hammers almost £6 million when he joined the club from AS Livorno almost exactly a year ago.
He went on to score eight goals from 29 games in a disappointing Hammers side last season. Despite a poor goals to games ratio, Diamanti was one of the few players that seemed to wear his heart on his sleeve.
If Brescia manage to avoid relegation this season then a further €300,000 will be paid to the Hammers.
Diamanti’s departure leaves a space open for Avram Grant to add to his squad before the transfer window closes at the end of the month.