Eagles' manager Ian Holloway confirmed on Saturday that he was still looking to add four or five players to his squad, following their first Premier League win of the season at home to Sunderland.
Reading winger Jimmy Kebe was the first of those new recruits, penning a deal late on Saturday evening, and he has now been joined by former Royals team-mate Mariappa.
The 26-year-old Jamaica international only moved to the Madejski Stadium last summer but could not prevent Reading slipping straight back down to the Championship.
He becomes Holloway's 13th signing of the window and is glad to be back in the top flight, signing a three-year deal at Selhurst Park.
"I'm delighted to be joining Palace and playing in the Premier League again. It's something everyone dreams of doing," Mariappa told the club's official website.
"Playing there last season was a great challenge as it is is the biggest league in the world and you're playing against the best players, so when this opportunity came to me I jumped at it.
'"Selhurst Park is always a great place to play, the atmosphere has always been amazing and I've had some great games against this club.
"I want to prove I can play at this level and I'm sure that will be the same for everyone in the squad as we aim to stay in the Premier League."
Scottish international Bannan, 23, had found first-team chances harder to come by at Villa Park in recent years and will link up with boss Ian Holloway for the second time, after playing under the Eagles boss during a loan-spell at Blackpool.
Bannan, who put off joining up with the Scotland squad to thrash out the terms of a three-year deal with Palace, told the official website he was looking forward to a fresh start at Selhurst Park.
"I went back home to Scotland over the weekend ahead of meeting up with the international squad so I've had to travel back down but it's a great bit of news for me," he said.
"I was at Blackpool with the gaffer here when they got promotion that year and that was probably the highlight of my career so far.
"It is a big move for me to leave Villa and I would like to thank the fans there but it's a new chapter in my life now and I can't wait to get started.
"I want to get myself in the door and start training and hopefully get myself in the team. I haven't played this season and didn't much last year so I can't wait to get going."