The figures are far higher than official data, once wider measures of joblessness across the UK are included, the union organisation said.
As well as the 2.5 million official jobless figure, a further 2.26 million people want a job but are not classified as unemployed, according to the TUC.
A report published for the TUC Congress, which opened in Bournemouth, said wider forms of unemployment should be given the same level of importance in the UK as in the United States.
TUC general secretary Frances O’Grady said: “Unemployment may have started to fall in recent months but we are still in the midst of a job crisis. The true scale of unemployment is far bigger than official figures suggest, as nearly five million people say they want work today.
“With a further 1.4 million people only able to find part-time work, despite needing a full-time job to get by, it’s clear that our labour market remains far from healthy.
“With the Bank of England now focused on unemployment, it’s important that official figures better reflect the true nature of the jobs market.
“We know that the recent fall in unemployment has been driven by short hours, low pay, temporary contracts, and jobs that offer no guarantee of paid work at all.”