The left-back has impressed since joining Saints last summer and was thrilled to complete a permanent move on Monday's closing day of the winter transfer window.
He told saintsfc.co.uk: "I wanted to sign and commit somewhere instead of carrying on doing loans
When I had the chance to sign here, it was the perfect match.
"I have really enjoyed my time here in the first half of the season and I'm delighted to make it permanent.
"Whether I'm on loan or permanent, I will give 100 per cent, no problem, but moving here gives me added incentive as well.
"Finally I can say this is my club and I'm proud to do that
The work starts here and I'm really looking forward to it."
Bertrand hopes to be successful individually and with Saints.
"I want to keep progressing as a footballer," he added.
"Hopefully we can progress as a team and go on to achieve some really great things
It's no fluke where we are in the league at the minute.
"We're playing to the best of our ability and striving to become better players
If we all maintain our hard work we can only go forward as a club."
Bertrand won the 2012 Champions League with Chelsea and earned England recognition, but found first-team opportunities limited.
The 25-year-old added: "I have very fond memories of Chelsea
I have a lot of people to thank at the club having been there since the age of 15
The fans were fantastic with me there from day one.
"We had a fantastic relationship so from that aspect it was hard, but to finally get to a team that I can say is mine and my own is a really big positive for me on a personal level.
"There's not one reason in particular why this was the club for me
The stability of the club and from what the owners are thinking right down to youth level is clear, honest and transparent.
"It's very important that the club shares the same ambition as the players
They are forward thinking, so are we as players, together that makes us a really strong unit.
"Everyone is pulling in the right direction and I wanted to be part of that."
Source : PA