The 26-year-old - who played 89 times for the Gunners before leaving to join the Saints in a £25million deal in August - was in tears during the warm-up.
Ramsdale then made three saves against his old side but that was not enough to stop Southampton falling to a 3-1 defeat.
Arsenal fans chanted Ramsdale's name towards the end of Saturday's match.
After the final whistle, Ramsdale was then seen exchanging words with Arsenal manager Mikel Arteta and current Gunners goalkeeper David Raya.
Towards the end of the match, Arsenal supporters serenaded Saints goalkeeper Ramsdale
Arteta was asked during a post-game press conference what he had said to Ramsdale.
He replied: 'I wished him all the best and congratulated him for the way that they played as well.
'I wish him the best and I hope they do [well] – especially for the manager and Aaron because they are a really good side.'
Arteta had also spoken about Ramsdale in his pre-match press conference.
Speaking to the media on Friday, the Spaniard revealed what the first thing he did was after finding out that Ramsdale had officially left the club in August.
'I called him straight away after I got the news [of his move to Southampton],' Arteta explained. 'He was really happy.
'He is a player we love a lot. Very charismatic. He really put his fingerprints here in the way he was. And it will be very nice to see him.'