The move comes two years after planned pension reforms to annuities led to a wipe out of sales.
Under the terms of the merger, Just Retirement will own 60% of the new firm, with Partnership Assurance shareholders owning the rest of the business.
The deal, first agreed in August last year, means the JRP Group has assets under management of more than £15bn.
Two years earlier, both companies suffered plummeting share prices and sales of annuities as George Osborne announced reforms granting unrestricted access to pension savings – scrapping the need for people to buy an annuity.
Coupled with low interest rates many people have since sought out alternatives to annuities such as drawing down income from their pension pots.
Annuity sales in the UK plunged 42% from £11.9bn in 2013 to £6.9bn in 2014. Just Retirement suffered a 56% fall in individual annuity sales in the year to June, while Partnership confirmed sales fell by half during 2015.
However, recent research published last week by the Association of British Insurers (ABI) found that annuities are beginning to bounce back after their post-freedom fall and are now almost on par with drawdown sales.
The data based on the last quarter of 2015, found that £3.3bn has been invested in around 61,700 annuities in the UK – compared with £4.2bn invested in 63,400 income drawdown products.
In a statement, JRP Group said it was still keen to provide products such as medically underwritten annuities for savers with health problems, as well as signing up companies for de-risking deals, which transfer the future cost of a firms’ pensioners into an insurer.
Former head of Just Retirement, Rodney Cook, who is now the chief executive of JRP Group, said: “I am delighted the merger process between Just Retirement and Partnership has been concluded and JRP Group is now established.
“As we launch, we are optimistic that our new company will use the outstanding intellectual property we have developed, together with our multi-channel distribution assets and market leading service to positively disrupt markets to deliver improved value to our customers.”