The 31-year-old comes in as a replacement for Danny Ward, who was recalled from his spell at Pittodrie by parent club Liverpool this week.
Collin, who joined Rotherham from Carlisle United in 2013, has played twice this season.
But he has not featured since August and recently found himself relegated to third choice.
Aberdeen manager Derek McInnes hopes to have Ward available to face Ross County in their Scottish Premiership game in Dingwall.
"We're still waiting for international clearance for Adam and I'll announce the team on Sunday morning," he said.
"We'd assurances from Liverpool that Danny could stay, but their circumstances changed, so we're delighted to get Adam on board."
Ward impressed during his spell with the Dons and, on his return to Anfield, was immediately handed a place on the bench for the game against Arsenal.
There had been suggestions that Aberdeen would consider recalling Danny Rogers from his loan with Falkirk and the Pittodrie outfit had also been linked with out-of-favour Rangers goalkeeper Cammy Bell.
"There was no approach for Cammy Bell, although I'm not saying his name wasn't mentioned," said McInnes.
New signing Collin began his career with Newcastle United and had loan spells with Oldham Athletic and Doncaster Rovers.
However, it was not until he joined Northern Premier League club Workington in 2004 that he made his senior debut.
McInnes would not comment on speculation linking Stuart Bannigan of Partick Thistle with a move to Aberdeen.
However, asked about the possibility of more signings during the January transfer window, McInnes admitted that injuries to Willo Flood and Ryan Jack had left his squad short in central midfield.
"We may be looking to bring one in and for nobody to go out," the manager added.
Meanwhile, McInnes also said any decision on the future of defender Paul Quinn will be made after Sunday's game with Ross County.
Quinn has been linked with a move back to the Dingwall club having joined the Dons from County in the summer.

Students from 12,000 schools nationwide will be told not to do their weekend homework for the month of November.
The Spanish Alliance of Parents' Associations (CEAPA), which called the strike, argues that homework is detrimental to children.
Spain was high up on a homework league table in a 2012 education report.
The PISA study, by the Organisation for Economic Co-operation and Development, found that Spanish children and teenagers have 6.5 hours of homework a week, compared with an average of 4.9 from a group of 38 countries.
Spain ranked 11th on a list of 64 countries or locations in a PISA table covering the amount of homework given.
The workload does not necessarily translate in better results for Spanish students - they have traditionally scored low results in maths, reading and science in other PISA reports.
By contrast, in Finland and South Korea, two of the countries with the best student performances according to PISA - the average time spent on homework every week was less than three hours.
So what does this say about the learning methods used in Spain?
CEAPA president Jose Luis Pazos says education in Spain has been very reliant on the traditional method of rote-learning - memorising work.
He stresses that what children have to learn is how to manage information and how to decide what to memorise and what not.
"Society has changed deeply, but the environment in the classroom hasn't," he says.
The homework debate has been raging in other countries too, and it is not just parents who are concerned.
In the US state of Texas, a secondary school teacher near Dallas sent a letter to parents telling them that she will not be assigning homework to students this year. Instead, she said she wanted them to spend more time with their families.
A high school in Britain scrapped traditional homework at the start of term in favour of a more "independent" approach to learning.
And even in South Korea, the authorities are considering reducing homework for younger pupils even further. The Yonhap news agency reported in August that the city education office in Seoul was banning primary schools from giving homework to lower grade students, starting next year.
Rotherham United goalkeeper Adam Collin has completed his switch to Aberdeen on loan until the end of the season.
[NEXT_CONCEPT]
Thousands of parents in Spain are going on strike against state schools this weekend over the large amount of homework given to their children.