CWU general secretary Dave Ward hailed him as "a leader for the millions, not the millionaires".
Thanking the union for its support, Mr Corbyn said it was "time to end rip-off Britain."
The announcement came as two former advisers declared their support for his rival, Owen Smith.
Academics David Blanchflower and Simon Wren-Lewis, who have both served on Labour's economic advisory committee, expressed doubts about Mr Corbyn's ability to lead.
Mr Blanchflower told The Guardian Mr Smith had been better at consulting businesses and economists in three weeks than the current Labour leader had been over the last nine months, and Mr Wren-Lewis wrote on his blog that "a Corbyn-led party cannot win in 2020".
Endorsing Mr Corbyn, the CWU general secretary said: "We need a fundamental change in Labour politics and Jeremy Corbyn recognises this.
"He is the candidate to drive through the change that ordinary people are crying out for - opposing damaging austerity measures and tackling the housing crisis which is causing misery for so many."
The CWU represents 190,000 people working in the Royal Mail group, mobile companies and in BT Open Reach.
Mr Corbyn has already received the backing of train drivers' union Aslef and construction workers' union Ucatt.
Meanwhile, Labour has announced the dates of five leadership hustings over the next month to be held in Cardiff (4 August), Nottinghamshire (17 August), Birmingham (18 August), Glasgow (25 August) and London (1 September).
The Communication Workers Union has announced it is backing Jeremy Corbyn in the Labour Party leadership election.