Fund managers Aberdeen Asset Management and Standard Life have agreed terms for an all-share merger.
The move will create one of the UK's largest fund managers, overseeing assets worth £660bn. The two companies employ about 9,000 people.
The companies said the deal was subject to a number of conditions, including shareholder approval.
It says the combined group will be named to incorporate the names of both Standard Life and Aberdeen.
Following completion of the merger, Aberdeen shareholders will own about a third of the combined group, with Standard Life shareholders owning the remaining two thirds.
That reflects Aberdeen's £3.8bn market capitalisation and Standard Life's £7.5bn value.
Standard Life chairman Sir Gerry Grimstone will become chairman of the board, with Aberdeen chairman Simon Troughton becoming deputy chairman.
Job loss predictions
Standard Life and Aberdeen's current chief executives, Keith Skeoch and Martin Gilbert, would become co-chief executives.
Lloyds Banking Group, which has a 10% stake in Aberdeen Asset Management, has backed the merger deal.
BBC Scotland business and economy editor Douglas Fraser said the logic of putting the firms together was to build investment clout, so they can compete with US-based giants such as Blackrock.
But he said analysts were predicting the merger could mean around 1,000 job losses - representing about one in 10 of the current employees.
He added: "A lot of them would be in back-office, in IT and the sales team.
"The answer we are getting from the joint company this morning is that they want to stress that they want to grow.
"They don't dispute that there are cost advantages and that jobs are likely to be shed, but they want to talk about the opportunities for growth such as Standard Life investments have seen in recent years."
Analysis from Douglas Fraser, BBC Scotland's business editor
This puts the companies in a strong position as a global player in asset management of funds. It gives them scale, it gives them a deep pool of expertise with which to research the companies they invest in.
Scale counts when you are talking information technology and computing, back-office, where there will be some duplication and they can cut costs there.
What they are not spelling out so much is that they are under competitive pressure because they are in the business of active management of funds. Money is pouring into passive management [funds run by computer that simply track a market] without the expensive research teams.
The new company in Scotland wants to make the case for competitively-priced active management as a means of getting ahead of the average market out-turn and, in particular, it is being driven by a relatively weak position for Aberdeen Asset Management.
A lot of funds have been withdrawn in the past few years. Its portfolio of investment in emerging markets, where it's had its real strength, has not performed that well because of the volatility in those markets. It did well while they were doing well but it is a volatile place to be.
The company has been steered for 34 years by Martin Gilbert, much of that by acquiring other funds, and it does seem it has run out of other options to move on.
Following concerns over job losses in the north-east, Gordon MP Alex Salmond said he had been "reassured" by Mr Gilbert, from Aberdeen Asset Management.
In a series of tweets, Mr Salmond said: Greatly reassured about outlook for jobs in Aberdeen... With emphasis on growing company and using workforce skills.
"Merger of @AberdeenAssetUK and @StandardLifeplc will have key benefit of creating world class Scottish investment house... With their respective highly attractive asset bases now secure from takeover."
He added: "Best wishes for the future success of the new world reach company."
A Scottish government spokesman said: "Aberdeen Asset Management and Standard Life are leading firms in Scotland's financial services industry and the proposed merger is a potential vote of confidence in Scotland's financial services sector.
"We will be engaging with both companies as the merger progresses to discuss employment and investment in Scotland. We welcome the intention to grow the business in Scotland and to build on the expertise and skills of both companies and strengthen Scotland's reputation for fund and asset management.
"The Scottish government has been in contact with both companies and as plans for the merger are confirmed the Scottish government stand ready to support the new business and their employees."