The personal trainer, who was born in Iran, seems to have been a stable force in her life since they met in 2016. Before her conservatorship ended in November, he said he was "looking forward" to a "normal, amazing future" with Spears, 40.
From standing by the Toxic singer's side as her conservatorship was dissolved, to his childhood in Tehran and Hollywood jobs, here, we tell you everything you need to know about Asghari.
He was born in Tehran
Asghari, 28, was born in Tehran, but moved to the US to be with his father when he was aged 12.
However, his whole family did not move together. He explained to Forbes in a 2021 interview: "I actually left my family behind at the time because my family was still going through the US immigration process.
"I left three of my sisters behind, my mum behind. My dad was out here in America and I hadn’t seen him in about five years or so. It was definitely a culture shock, coming out here with a completely different language to speak."
He told Men's Health magazine in 2018: "I had to leave my friends and family who I was very close with, and I knew I was moving away and never coming back."
He moved to Los Angeles, California, where his father was working as a truck driver.
He could not speak English when he moved to the US
Of the "culture shock" of moving to the US, Asghari explained to Forbes: "I didn't speak a word of English and it was a weird age where I was finally so comfortable with my friends and [I had to] leave them behind to start a new life.
"So when I got out here, it was very different, but I always had a positive mindset. I always knew that I was coming to a country where opportunity was the equivalent to gold. I was brought up by my dad knowing and understanding that we were coming here for a better life."
He is keeping Iranian traditions alive in the US
In March 2021, Asghari shared a video of himself teaching Spears a Nowruz greeting.
In the video, he repeats the phrase "sale no mubarak", with the Oops! ...I Did It Again singer repeating it back to him, without perfecting the pronunciation each time.
In the end, he says, "Alright, just say 'Happy Persian New Year'".
He met Britney Spears on the set of her music video
The couple met in 2016, on the set of Spears' Slumber Party video. Asghari plays the singer's mysterious love interest in the music video, which is set in an old mansion that is being used as a nightclub.
Of getting the gig, he told Forbes: "A good friend of mine was working on a project and they referred me to the team that was picking out the leading role for Slumber Party. My girlfriend now, at the time, personally she picked my picture and she wanted me to be cast in the music video.
"My friend called me and said, ‘I need you to be on it. Trust me, you want to show up.’ I didn’t know who was shooting. It was a secret project. So I showed up because of my friend. I showed up and everything just kicked off from there."
He has also described having "butterflies" before meeting Spears on set, telling Men's Health: "I was excited to meet one of the biggest artists of all time."
He was also in a Fifth Harmony music video
The Slumber Party video is not the first video Asghari starred in, as he had previously featured in Fifth Harmony's music video Work From Home.
"I was already known on a music video, so I didn’t want to do another music video because of that," he told Forbes. "I wanted to do TV, I wanted to do film. My acting strategy was kind of to go away from music videos."
He has Hollywood aspirations
In May 2021, Asghari revealed his ultimate career ambition is to become a superhero.
The actor and Instagram star opened up about his career goals in a Variety interview, saying that Marvel should "give him a call".
Asghari has been slowly making a name for himself in television, starring in HBO Max's Hacks, as well as landing a recurring role in season three of Showtime comedy series Black Monday.
However, comedy is not his ultimate goal, he said in a chat with Variety.
"Action is something that I want to do — action, drama, thriller — that’s a genre that I want to really get into,” he said. “But if you can do comedy, you can do anything."
He also said he gets his inspiration by studying the craft during his intensive cardio sessions at the gym.
“I want to get into blockbusters, I want to get into TV shows that get on Netflix and become fan favourites,” he said.
“My ultimate goal is to really become a well-rounded actor. I’m doing a lot of MMA training, I’m doing a lot of gun and stunt training to be a well-rounded actor such as Tom Cruise, such as Jason Statham.
"Like I said, I want to do action, but I want to be able to do drama as well. This is my craft, and I want to accomplish it 100 per cent.
“Marvel or anybody – they should give me a call.”
He was outspoken out about ending Spears's father's conservatorship
"I have always wanted nothing but the best for my better half, and will continue to support her following her dreams and creating the future she wants and deserves," he told People magazine. "I am thankful for all of the love and support she is receiving from her fans all over the world, and I am looking forward to a normal, amazing future together."
He also took to Instagram to voice his opinion about the pop star's father.
“Now it's important for people to understand that I have no respect for someone trying to control our relationship and constantly throwing obstacles our way,” he wrote. "I won't be going into details because I've always respected our privacy, but at the same time I didn't come to this country to not be able to express my opinion and freedom."
He has been open with his plans to have children with Spears
The singer is mum to Sean Preston Federline, 16, and Jaden James Federline, 15, and is currently expecting her first child with Asghari.
"Marriage and kids are a natural part of a strong relationship filled with love and respect," he wrote next to a watercolour painting of a family of lions.
"Fatherhood is something I have always looked forward to and I don’t take lightly. It is the most important job I will ever do."
He also spoke about plans to have a family with the singer in the past.
"My priorities in life are to remain humble and understand where I came from and where I'm going," he said during his 2021 Forbes interview.
“I want to take my career to the next step when it comes to acting. I want to take my relationship to the next step as well. I don’t mind becoming a father. I want to be a young dad.”
He proposed with a personalised $70,000 engagement ring
Spears announced her engagement Asghari in September.
Showing off her sparkling new ring, the singer captioned an Instagram video, "I can't believe it," with a series of diamond ring emojis.
Details about the bespoke piece of jewellery were revealed by the designer, Roman Malayev, at Forever Diamonds NY. The ring is estimated to have cost $70,000 and has the word "lioness" engraved inside, which is Asghari's nickname for his new fiancee.
"The thought and detail Sam put into this ring made the design process so special for Roman [Malayev] and our team. In honour of the lioness herself, we are naming this gorgeous setting The Britney. Sam and Britney, we wish you a lifetime of happiness together," a post on the Forever Diamonds NY Instagram account reads.
The Britney ring is now a permanent part of the Forever Diamonds NY collection. It features a four-carat round brilliant stone in a platinum cathedral setting, with pavé detailing on the bridge and flush-set diamonds on the prongs and basket.
He is a personal trainer and social media influencer
Asghari regularly posts fitness videos online with Spears, which he says are her idea. He has 2.4 million followers on Instagram, who he shares the videos with.
"The exercises, I come up with them, but the videos are because of my better half. She's the video director," he told Men's Health. "She motivates me more than anyone. It's crazy how I can be humbled by a person like her. If I could be that humble when I'm that high in life, that would be the best thing. She is another blessing that happened to me."
He was not always super-fit
Asghari has been honest with his past battle with his weight, saying he weighed 132 kilograms at one stage. However, in 2013 he decided to focus on getting into shape, losing 45kg.
In a 2020 Instagram post, he opened up about being bigger and getting into shape, writing: "As a former member of the 'chunky boy friend-zoned club' the love for cheesecakes was real. The anticipation for taco Tuesday on Monday nights, the butterflies, the sense of nervousness felt like a professional athlete before game days, so I decided to make taco Tuesday every day and naturally I gained massive weight."
He added: "After high school, I became very close to obese, but I was fortunate to stop and realise that I came to this country as an immigrant to achieve phenomenal success, and be the best version of myself. I understood that I was a victim of self sabotage, so I decided to change my lifestyle all around."