Following court approval, the deal has been completed and JRP Group plc has been formed.
The company has a market cap of £1.4bn and assets under management of more than £15 billion.
Rodney Cook, group chief executive, said: “I am delighted the merger process between Just Retirement and Partnership has been concluded and JRP Group plc 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.”
JRP Group said it was “focused on utilising its improved scale, efficiency and capital strength to continue providing customers with better value alternatives to products offered by large incumbent insurers”.
The Just Retirement board has said it expects the merger to result in pre-tax cost savings of at least £40 million per year, with the full run-rate being achieved in 2018.
One-off integration costs of £60 million over two years have been forecast.
Tom Cross Brown, chairman of Just Retirement, previously said: "This transaction represents a unique opportunity to accelerate the existing strategy of both businesses. Our two businesses will be bigger, stronger and more efficient together, which we believe will allow us to deliver better returns to both policyholders and shareholders."
A previous statement from the companies read: “The boards believe that the merger will deliver significant strategic and financial benefits for the combined group.
“The larger capital base will enable a broader defined benefit proposition and enhance the group's perceived strength of covenant, opening up opportunities in the attractive defined benefit scheme de-risking segment.
“The merger will strengthen the competitive position of the combined group in the UK retirement income market, expected to lead to improved customer outcomes compared to the products currently offered by larger incumbent insurers.”