The couple, who began dating last year, are yet to set a wedding date.
In a statement issued by Buckingham Palace, it said: "Both families were informed jointly of the announcement and were delighted with the wonderful news of their engagement."
Harriet has joined the royals at a number of high-profile engagements over the past year, and seems to have been invited warmly into the Firm.
WATCH: Mike Tindall recalls hilarious blunder in front of Princess Anne
Princess Anne appears to have a close relationship with her son-in-law, Mike Tindall, and her former daughter-in-law, Autumn Phillips.
Here are some of favourite photos of Anne in mother-in-law mode.
Just weeks before their engagement announcement, Harriet and Peter attended Royal Ascot. Anne was seen greeting her future daughter-in-law with a kiss.
On their second outing at Royal Ascot in June, Peter and Harriet made their carriage debut in the royal procession. Cameras captured the moment Zara and Anne reached out to embrace Harriet as she stepped off the carriage.
Just months before his wedding to Zara in 2011, former England rugby star Mike was presented with the Calcutta Cup by Princess Anne.
The trophy is famously given to the winning team at England and Scotland rugby matches, and with Anne being the patron of the Scottish Rugby Union, Mike later revealed there was a cheeky exchange between the pair as he joked about "getting one over" his mother-in-law.
Mike seems to have a fun relationship with his mother-in-law. Speaking about marrying into the royal family, he told The Telegraph in 2021: "I can only say how kind they've been to me, and how welcoming they've been to me since joining the family. And how they've made my family welcome."
In 2023, Anne agreed to go on Mike's podcast, The Good, The Bad & The Rugby, alongside the Prince and Princess of Wales. The episode shared an insight into their family dynamics as well as their shared love of the sport.
"They were amazing," Mike said of his in-laws before explaining why he wanted to bring them onto the podcast.
"Rugby has been going through a tough time and we wanted to showcase why it brings people together from all backgrounds, it's such an inclusive sport, but it also creates great human beings," he said.
"It was more important to showcase why they like sport, why Princess Anne has been a patron of [the Scottish Rugby Union] for 37 years and why she loves the characters in the game.
After separating from Peter in 2019, Autumn has remained living close to her daughters in Gloucestershire. The Canadian businesswoman joined Anne to support Zara at the Cirencester Park horse trials in April.
Royal photographer James Whatling previously told HELLO!: "It was just nice and relaxed.
"Zara had a group of pals and grooms with her. Autumn was there, which was nice to see. She's obviously still part of the family and was chatting to Anne and everyone else.
"She's certainly not been ostracised in any way, and it was a nice little support team for Zara. They were all following her around to each section – dressage, showjumping etc."
It's always a good time to write about Princess Anne but with the hardworking royal turning 75 next week, the timing is particularly perfect. Royal Editor Emily Nash has been fortunate enough to get up close and personal to Anne on several occasions, and in her Friday newsletter reveals seven things she has learnt about the King's loyal and lighthouse-loving sister. You have to be a member of The HELLO! Royal Club to read the full article, but it's well worth it!
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The former rugby player refrained from commenting after his The Good, The Bad & The Rugby co-host, James Haskell, joked about Andrew Mountbatten-Windsor.
