The Labour MP for Ashton-under-Lyne in Greater Manchester, replaces Pat Glass, who resigned on Wednesday, two days after she was appointed by party leader Jeremy Corbyn.
Ms Glass took over from Lucy Powell, who tendered her resignation on Sunday, June 26.
The news follows a string of Labour frontbench resignations, along with the Parliamentary party’s vote of no confidence in Mr Corbyn, who lost out 172 votes to 40.
Ms Rayner, whose husband Mark is a full-time trade union official, was elected as an MP following the 2015 General Election.
Before her entry into Parliament, she worked as an elderly care worker and a trade union official.
In her maiden speech to Parliament, Ms Rayner said, ‘A care worker becoming an MP - that's real aspiration for you.
'Perhaps too, I’m the only member of the house, who at age 16, and pregnant, I was told in no uncertain terms, that I would never amount to anything.
‘If only they could see me now.’
She has performed the roles of opposition whip and shadow pensions minister, and served on the communities and local government committee between July and October last year.
Ms Rayner hit headlines in November last year, after sending a written complaint to a shoe shop on House of Commons-headed paper.