The Langley Ward and Belvoir Unit at the Glenfield Hospital site will close until April along with the nine-bed Grasmere Unit at Rowletts Hill.
Barry Day, deputy chief executive of the Leicestershire Partnership NHS Trust, said the trust had to claw back a £1.98 deficit by the end of March.
Health service union Unison said it was saddened by the closure.
'Another cut'
Mr Day said the situation would be reviewed weekly and staff from all three services were to be redeployed across the trust.
"It isn't comfortable for some families and some of our patients and we feel for them in these circumstances, but what we're trying desperately to do is minimise any disruption to them and their care," he said.
Unison representative Norma Burford said: "We've got staff and clients who use that service, all of whom will be affected in a negative way.
"That's another cut in services to mental health patients or those with learning difficulties in Leicestershire. Where's it going to end?"
The trust has also made an agreement with other health authorities to make a £500,000 contribution to ease the overall local health economy.
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