Most of the women wore red coats and aprons, and the men overalls and hard hats - to show they intend to represent the interest of workers.
The Economic Freedom Fighters (EFF) has 25 MPs in the 400-member parliament elected on 7 May.
It was the first election contested by the EFF, led by the firebrand Julius Malema.
He formed the EFF last year following his expulsion from the governing African National Congress (ANC) in 2012.
The EFF won 6% of the vote after campaigning on a promise to partially nationalise mines and seize white-owned farms.
The BBC's Pumza Fihlani in South Africa says its MPs made quite a statement in parliament in Cape Town with their outfits.
Other MPs wore traditional suits and fancy dress.
Outside parliament, EFF MPs sang and danced around the bronze bust of South Africa's first black President Nelson Mandela, as photographers snapped pictures.
"The working class and poor now know they've got representatives in parliament. There is no other organisation which associates itself with the struggles of the poor," EFF MP Floyd Shivambu said.
Our correspondent says the EFF is expected to contribute to robust discussions in parliament, but it does not have the numbers to deliver on its radical promises.
The ANC won the election with 62% of the vote, followed by the centre-right Democratic Alliance (DA) with 22%.
The ANC has governed South Africa since apartheid ended in 1994.
The parliamentary session was held to swear in MPs. President Jacob Zuma is due to be inaugurated for a second term at a public ceremony on Saturday.
South Africa's new left-wing MPs attended their first parliamentary session dressed as maids and miners.