The two charities will work together as part of the RLSB Group while preserving their separate identities, although a new logo that reflects both organisations is planned for later in the year.
The RLSB will continue to concentrate its work in London and the south east of England, while the Royal Blind Society will work across England and Wales.
The organisations work with a total of about 2,000 beneficiaries, but aim to double this number over the next four years.
Caroline Armitage, chief executive of the Royal Blind Society, will leave her job next month and act as a consultant to the new RLSB Group.
Eileen Harding, currently a corporate team director at the RLSB, will take over as chief executive of the Royal Blind Society.
The Royal Blind Society owns two hotels – one in West Sussex and one in Llandudno, north Wales – that offer holidays to visually impaired people and their families.
The charity said it was considering closing the Llandudno hotel and is consulting 20 staff over possible redundancies.
The RSLB’s income was £5.5m in the year to December 2012 and it employs 100 staff. The Royal Blind Society had an income of £1.2m in the year to September 2012 and has 40 employees.
The charities said that the move could result in a full merger, but beneficiaries were best served by both organisations retaining their identities for the time being.
"We will work hard to preserve the history of both organisations until being separate is less beneficial to those we serve than a full merger," a spokeswoman for the charities said.
Both organisations hope to make savings by combining back-office and fundraising functions but said no further redundancies were planned in these areas.
"This association will mean two of the oldest sight-loss charities in the country connecting as one strong innovative voice to realise our ambitions to drastically improve the life chances of the young people we serve," said Tom Pey, chief executive of the RLSB.
Armitage said she was delighted to be continuing in her post until Easter and would be "helping to ensure the smooth transition in an advisory capacity after that time".