Hearing Link will become part of Hearing Dogs for Deaf People from 1 August, the charities said in a statement, but both brands will continue to operate.
The merger will provide people who have hearing loss with access to a broader range of services, the statement said.
Hearing Link, which helps people to deal with the practical and emotional challenges of hearing loss, has been spending hundreds of thousands of pounds more than its income in recent years, accounts filed with the Charity Commission show. It spent £1.2m in 2015 against an income of £671,257, the accounts show.
"As with many small charities, the challenging and competitive fundraising environment has proved extremely difficult for Hearing Link," a statement on Hearing Link’s website said.
"By merging with Hearing Dogs and incorporating Hearing Link as a distinct service within the larger charity we have secured our future to enable us to continue to deliver our life-changing services."
Hearing Link has 13 staff. Hearing Dogs, which provides assistance dogs for people with hearing loss, has 184 staff and had an income of £7.5m in the year to the end of March 2016.
A spokeswoman for Hearing Dogs said the merger would not result in any redundancies.
Michele Jennings, chief executive of Hearing Dogs, will continue as chief executive of the merged organisation, and Lorraine Gailey, chief executive of Hearing Link, will become chief operating officer and retain responsibility for Hearing Link within the merged charity.
The spokeswoman said the charities would be able to make savings by merging back-office functions because Hearing Link would no longer need to outsource areas such as finance or IT.
Hearing Link’s board will be disbanded, but it is expected that some of the trustees will join Hearing Dogs’ board as the terms for existing members come to an end.
Jennings said: "The merger of our two organisations has secured the future of Hearing Link, which has found it increasingly difficult to maintain financial sustainability.
"The wealth of knowledge and expertise within Hearing Link will now be made available to even more people through the development opportunities provided by the combination of our more than 100 years of experience in helping deaf people."