An inspection found an increase in the number of these establishments in the city performing poorly.
And the condition of three homes was so serious they could have been closed down.
Around 4000 older people currently live in 85 care homes in Glasgow, costing the city council £54 million a year.
Last year, the Care Inspectorate checked the 54 homes run by the private sector and the 16 operated by charities.
Homes are awarded grades ranging from one – 'unsatisfactory' – to six 'excellent'.
They are graded on the quality of care and support, the environment, staffing and management and leadership.
In October last year, two homes were rated 'unsatisfactory or weak' across all four inspection categories and 11 were rated 'good and excellent'.
The number rated unsatisfactory and weak for quality of care and support rose from 4% to 11% compared with 2011.
And the number of homes with 'good' grades had fallen from 42% to 28%.
David Williams, the council's executive director of social work, said: "In part, this deterioration can be attributed to changes to the inspection process which in some cases has resulted in lower grades than before for equivalent quality services.
"However, there is also evidence that a real deterioration in service quality has contributed to this.
"Although a clear majority of care homes in the city continue to provide high standards of care, over the last 12 months there has been a notable increase in the number of care homes in Glasgow that have performed poorly.
"The situation in three care homes – Broomfield Court, Ashton Grange and Torbrae – has been so serious that the inspectorate has issued formal improvement notices."
An improvement notice can be served when the organisation has "sufficient concerns" about a home.
It requires the home owner to make the required improvements in a given timescale. Failure to do so could result in the home closing.
Mr Williams added: "Broomfield Court's performance has returned to satisfactory levels. The other two homes remain subject to improvement notices.
"The issues experienced in Glasgow over the past year can be seen in the wider context of an unstable UK care home sector, encapsulated by the collapse of Southern Cross Health- care in 2011 and continued concerns about the financial wellbeing of the remaining large-scale care home providers."