The 23-year-old right-back moved to Liverpool from Aberdeen four years ago in a deal worth an initial £4.2million, but has made just four first-team appearances for the Anfield club since, with his time on Merseyside repeatedly disrupted by injury.
Loan spells at Preston North End, Bolton Wanderers, Wigan Athletic and Kilmarnock have brought a further 26 appearances combined.
Speaking to stmirren.com, Ramsay described his excitement at the move.
"I feel really good, really excited. I've been in and trained and met all the boys, so yeah, I'm looking forward to it. I know the league and I wanted to come back up here and try and show what I can do and rebuild myself again. I feel like it's a good place to come. I want to try and perform, work as hard as I can and see where that takes me," he said.
The Scotland international, who won the SFWA Young Player of the Year award during his breakout season at Pittodrie, also praised the club's facilities and the pitch at St Mirren Park.
"It's a really tough place to come for opposition teams and I think the pitch is really good as well from when I played here. It's a good set up, it's a good club and I'm just excited to get going," Ramsay said, adding that conversations with head coach Craig McLeish had convinced him the club had changed for the better.
"I spoke to the manager a couple of times before I came up and he got his plan across and what he wanted to do. It's probably a different St Mirren compared to the last few years. That'll suit me. I'm comfortable on the ball, I'm composed, I like to get on it and try and create things and make things happen."
McLeish, who has now completed his tenth signing of the summer, was equally enthusiastic about landing a player of Ramsay's calibre.
"We're delighted to have a player of that calibre join us. He brings a lot of quality to the group. He can play that position in a number of different ways and it adds a different dynamic to what we're doing," the St Mirren boss said. "It's a really important year for Calvin where he needs to find regular football to help kick on his career and we're happy to give him that platform because we get an exceptional young player to add to our group."
McLeish also addressed concerns over Ramsay's fitness given his injury-disrupted spell at Liverpool.
"He's had a lot of game time with Liverpool over pre-season. He's had the full pre-season under their new manager, which he said has been really intense. He said even just normal training sessions have been really intense too, so we've got no real issues there about him coming straight in."
No contract extension was agreed alongside the loan, meaning Ramsay's Liverpool deal will run down alongside the St Mirren agreement, with the defender potentially free to negotiate a permanent move elsewhere next summer.