Belardinelli, who will report to EMEA head of comms Gareth Mead, announced the news in a tweet on Friday afternoon.
Belardinelli was also a special adviser to Balls during his time as secretary of state for children, schools and families, and then handled Balls' press during his unsuccessful bid to become Labour leader in 2010.
According to his LinkedIn profile, Belardinelli has also worked for the charity Child Poverty Action Group and as national campaigns and membership officer for the Labour Party's student arm.
Last month, it contacted ad agencies in the UK, having moved its PR in-house with the hire of Mead earlier in the year. It is also currently looking for a consumer PR executive who would report into Belardinelli.