Matthew James, 30, from Trehafod, near Pontypridd, used his body as a human shield while trying to protect his fiancee when the gunmen opened fire.
At least 38 people died in Sousse with the Foreign Office confirming at least five were British.
Islamic State extremists have said they carried out the attack.
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Gas engineer Mr James' fiancee Saera Wilson, 26, said he was hit in the shoulder, chest and hip.
Speaking from the hospital on Friday where Mr James, known as Sas, is being treated, she said: "He took a bullet for me. I owe him my life because he threw himself in front of me when the shooting started.
"He was covered in blood from the shots, but he just told me to run away.
"He told me: 'I love you babe. But just go - tell our children that their daddy loves them'.
"It was the bravest thing I've ever known. But I just had to leave him under the sunbed because the shooting just kept on coming.
"I ran back, past bodies on the beach to reach our hotel. It was chaos - there was a body in the hotel pool and it was just full of blood.
"You just can't explain how terrible it was. It was chaos with screaming and gunshots. I'm just so glad Matthew is alive because so many other people are dead."
Gas engineers and friends of the couple have raised thousands of pounds to help support them and their two children while Mr James recovers from his injuries.
Several other tourists from Wales have also been caught up in the attacks.
Amanda Roberts, from Swansea, was staying with her family near to the affected Imperial Marhaba Hotel in the Port El Kantaoui district.
They were on the beach when the attack took place and she told the BBC: "Someone said 'run for your life'."
It is understood the family have returned to the UK.
Ms Roberts daughter Leiha Shaw, 28, posted on social media they had 15 minutes to pack, leave the hotel and get on a plane.
"So relieved to be home in the UK," she posted on Saturday morning.
"At the moment we are being looked after by officials at Manchester Airport who have been wonderful, I cannot thank them enough. Waiting now for them to get us home.
"(I) can't even put it in to words the feeling that I have today and what myself and family have been through. A very sad day for Tunisia and a very sad day for the world."
A number of Britons issued desperate pleas for information about relatives who have not made contact since the attack.
Chris Spiteri, 29, from Cardiff, said he was deeply worried about his 25-year-old cousin Lena Tanti, also from Cardiff, who is currently in Tunisia.
However, he later managed to speak to her and found out that she was at a different resort some two hours away.
Kelsie Collins, from Pontypridd, was also on holiday in Sousse with her family and described scenes of confusion and fear. Her grandmother, Mavis Collins, spoke of her relief at hearing the family is safe.
Stacey Webb, 23, from Barry, said she was locked in a bathroom with seven others following the shooting.
"I have never been more scared in my life but I hope God is with me", she said.
Wrexham FC footballer Wes York, 22, left Sousse just 48 hours before the killings.
The striker told the Daily Post he would have been at the targeted beach, had he not cut his holiday short to prepare for pre-season training.
"I have been watching what has happened on TV and it just sends chills down your spine to think I was only just there," he said.
Tunisia has been on high alert since March when militants killed 22 people, mainly foreign tourists, in an attack on a museum in the capital Tunis.
A suicide bomber blew himself up in a failed attack on the beach in Sousse in October 2013.
Tunisia's prime minister Habib Essid has said the majority of the 38 people killed in the attack were British.
Speaking in Downing Street after chairing an emergency Cobra meeting on the UK's response to the attacks, Prime Minister David Cameron said he could only imagine the "pain, and anguish and agony" of families caught up in these "terrible events".
Thomson and First Choice said the two hotels where the attacks took place - the Hotel Riu Imperial Marhaba and the Bellevue - were part of their programme.
The companies said their customers were among "a number of fatalities", and they had sent 10 planes to bring home 2,500 tourists.
All Thomson and First Choice holidays to Tunisia have been cancelled for the next week.
The Foreign Office's helpline number is 0207 008 0000.
A Welsh tourist who was shot three times during the terrorist attack on a Tunisian beach is due to be flown home, BBC Wales understands.