The union "overwhelmingly" voted to support remaining in the EU at its annual conference.
The news comes after Unison officially backed Remain ahead of the referendum earlier this month, following Unite and the GMB union.
Following the vote in favour of backing campaigning to stay in Europe, Dave Ward, CWU general secretary, said: "We’ve had enough of the Boris and Dave show. This referendum is about our jobs, our rights at work, our homes, our public services and the prices we pay in the shops. We cannot let it be hijacked by a right-wing agenda.
"A vote to leave would fire the starting gun on a race to the bottom on employment rights which only the EU has stopped the Tories from scrapping. Promises of sovereignty are meaningless when working people will be denied a voice in the workplace.
"The EU is far from perfect - but it's necessary for tackling inequality, tax avoidance, climate change and preventing workers being exploited across Europe. It's time for the labour movement to make the progressive case for staying in the EU. This referendum is not a contest between two Tory visions for our future."
The announcement was seized upon by Britain Stronger in Europe, the officially designated Remain campaign.
Will Straw, executive director of Britain Stronger In Europe, said: "I am delighted that the CWU have thrown their support behind the campaign to keep Britain in Europe.
"CWU members and all working people are stronger in Europe, as trade with the EU creates jobs and keeps prices lower, while EU law guarantees crucial rights at work like maternity and paternity leave, paid holidays, and equal treatment for agency workers. Leaving Europe would be a leap in the dark that would put jobs and workers’ rights at risk."
The development is also a boost for the workers' rights argument for staying in the EU, especially in light of Labour leader Jeremy Corbyn's backing of membership.
Alan Johnson, a former general secretary of the union, said he welcomed the "overwhelming" vote for remain. "As a former postman and CWU General Secretary, I know the vital difference that union members can make to this campaign," the Labour In for Britain chair added.
"The decision by CWU members highlights the unity of the Labour movement with all the biggest affiliated unions now campaigning for Britain to remain in the EU. Labour is united in campaigning for Britain to remain in Europe because of the jobs, growth and investment and protections for British workers and consumers that depend on our EU membership."