The rugby player, 29, left singer-songwriter Layla Manoochehri, 25, heartbroken after opting for the Middlesex model, 24.
After turning down Layla, who famously used to date Blue singer Simon Webbe, Henson declared his love for bubbly blonde Carianne.
You're The One! Gavin Henson presents Carianne Barrow with the final red rose after choosing her to be his girlfriend on The Bachelor
I love you: The rugby player and model both declared their love for each other
He told her: 'I've been looking for a long-term relationship. Someone I can give 100 per cent to and will get that back...
'Carianne, you're The One. I feel so lucky. I feel so lucky to have found you. you're so unique.
'Please give me the opportunity to make you feel special, Let me be your best friend and give me the opportunity to make you happy for the rest of you life.'
A rose for my rose: The couple embrace after deciding to be official
As she beamed with happiness, he then uttered those three little words - despite his sexy hot tub kissing session with Layla the night before.
He declared: 'I love you Carianne. Will you please accept this rose and make me the happiest man alive by being my girlfriend.'
As the new couple embraced, an emotional Carianne replied that she loved him too: 'I want to be with you forever.'
It's not you: Henson told runner-up Layla Manoochehri he had stronger feelings for Carianne
Over the past two months, the Welsh athlete has whittled down 25 women to just two for the grand final in St Lucia.
At the beginning of the show, he admitted he'd had some tricky moments with both finalists.
He said: 'They've stood out from the beginning. There has been a few hiccups with Carieanne and a bit of blip with Layla.'
Beautiful location: Henson told Layla it was over against the stunning background of St Lucia
He was helped on his way to his decision by his parents Alan and Audrey and younger sister Sarah, who had private conversations with each girls without Gavin present.
Layla assured his family she could cope with life in the public eye, alluding to her six-year relationship with Blue boy band singer Simon Webbe, which ended this year.
His father Alan wanted to know if Layla would give him some more grandchildren in addition to Dexter and Ruby - Gavin's two kids with ex-fianc?e Charlotte Church.
Your loss: Layla looked disappointed as she walked away from the rugyb player
He asked her: 'Do you have children...? Can you have children? Would you mind out if they turned out to be Welsh? Wales needs some more boys...'
However, Carianne admitted she didn't have much experience with children so would have to learn how to look after Gavin's son and daughter.
She said: 'I'm rubbish with kids... Once I get used to the idea, I should be alright.'
Reporting back to Gavin, his mother Audrey said: 'Carianne seems more naive than Layla. Layla is more like you, she's seen the world a little bit.'
Charming the family: Layla met Gavin's parents Alan and Audrey and younger sister Sarah before he made his big decision
After Gavin informed his family that Carianne had a degree so wasn't as dizzy as she came across, his mum replied: 'So she wouldn't be hanging on your coat-tails, then?'
Turning back to Layla, Audrey said: 'She reminded me a little of you. She's a confident girl. She's comfortable in her own self.'
However, Sarah said she would Layla a bit predicable: 'She's got a maturity for her age. But I felt like every question I asked, I knew what she was going to say.'
Getting on side: Carianne told Gavin's family they were a 'good-looking bunch'
Alan told Gavin Layla reminded him of Stacey Branning from EastEnders.
Before making his decision, he took each contestant on one last date to help him make their decision.
After a stroll in the Botanical Gardens and dinner, Gavin and Carianne took turns in massaging each other.
Following their pamper session, Carianne admitted she was struggling to control her lust: 'I did enjoy massaging him, I like touching his body but I don't think I got him lovestruck from my massage.
Romantic: The couple enjoyed a dip in the sea for their last date
Pampering: Gavin and Carianne ended their date by giving each other massages
'When Gavin was massaging me I felt a bit frisky. I feel like what's going on between me and Gavin is the beginning of something great.'
On his day with Layla, the pair enjoyed dinner in his private suite, before moving on to the hot tub.
Layla decided to make her move and the pair enjoyed a passionate kissing session in their swimwear.
Later, Layla hinted there had been more than kissing: 'What happened under the bubbles I couldn't possibly tell you.'
Passionate: After a romantic dinner for their last date, Layla and Gavin headed into the hot tub
But before asking Carianne to be his girlfriend, Gavin told Layla he simply had stronger feelings for Carianne.
He said: 'From the first moment I saw you I was attracted to you. I felt the connection straight away. I can be myself around you. I feel so comfortable around you. I'm always so excited to see you. You have the qualities that I value and are so important to me.
'You're such a beautiful person. But I'm so sorry I have stronger feelings for someone else. I have stronger feelings for someone else. It does hurt.'
Making her move: Despite his 'love' for Carianne, Gavin was quite willing for a kissing session in the jacuzzi with Layla