The 24-year-old will cost the Red Devils at least £48.6m potentially rising to £57m with performance-related add-ons, and the Red Devils are no strangers to splashing the cash on centre-backs.
The likes of Kalidou Koulibaly, Sven Botman and Nayef Aguerd also cost a pretty penny to bring to the Premier League, but where does Martinez rank among a selection of revered names?
Here, Sports Mole counts down the 10 most expensive transfer fees for defenders of all time following confirmation of Martinez's move to Old Trafford.
10. Ben White - Brighton & Hove Albion to Arsenal - £52.7m
Eyebrows were raised and jokes were made when Arsenal forked out more than £50m to sign Ben White from Brighton in 2021, but the England international is right at home in the heart of the Gunners' defence.
The 24-year-old formed an effective partnership with Gabriel Magalhaes at the back last term, making 37 appearances in all tournaments and keeping 13 clean sheets in the Premier League.
Also lauded for his ability to carry the ball out from the back and progress play up the field, White should only be even more motivated to improve now that William Saliba has arrived to add greater competition to the centre-back ranks.
9. Lisandro Martinez - Ajax to Manchester United - £57m
Arsenal could very well have added Lisandro Martinez to the books this summer, but the pull of working with Erik ten Hag again at Manchester United was too strong for the Argentine to ignore.
Martinez won the 2021-22 Ajax Player of the Season award under Ten Hag, scoring once and providing four assists in 36 appearances, and his versatility should also come in handy this year.
Much has been made of Martinez's 5ft 9in frame, which may not serve him very well as a Premier League centre-back, but utilising the 24-year-old at left-back or in midfield could bear fruit for the Red Devils.
8. Aymeric Laporte - Athletic Bilbao to Manchester City - £58.5m
Another example of a big-money success story, Aymeric Laporte was a rare exception to Athletic Bilbao's unwritten policy of only signing players who had Basque heritage after joining their youth setup in 2010.
The La Liga side became £58.5m richer after letting the French-born Spain international depart for Manchester City in 2018, and Laporte has since ended four of his five seasons at the club with a Premier League winners' medal around his neck.
Laporte struggled to fend off competition from John Stones in 2020-21, but he is firmly Ruben Dias's first-choice partner again and recently ruled out the prospect of an exit from the Etihad anytime soon.
7. Achraf Hakimi - Inter Milan to Paris Saint-Germain - £59.9m
Real Madrid could have done with an Achraf Hakimi amid Dani Carvajal's recent injury woes, but they instead allowed the right-back to head for Inter Milan and have a direct hand in 17 goals during their 2020-21 Scudetto triumph.
However, Inter's volatile financial situation would see Paris Saint-Germain take advantage with a £59.9m swoop, and the goal contributions did not dry up for the Morocco international in Ligue 1.
Hakimi scored four goals and set up six more in 32 top-flight appearances for PSG last term and will seemingly retain his spot as the club's first-choice right-back under Christophe Galtier, although his agent has been forced to bat away rumours that the 23-year-old is unhappy in Paris.
6. Joao Cancelo - Juventus to Manchester City - £60m
Man City may have only paid an upfront fee of £27.4m to sign Joao Cancelo from Juventus, but Danilo being sent in the opposite direction brought the total value of the exchange to £60m, and he has been worth every penny.
The 28-year-old did not set the world alight when he first arrived in Manchester in 2019, but he has now been named in the PFA Team of the Year two seasons running and set up seven goals in the Premier League last term.
Whether he is beefing up the City defence at full-back or contributing to the attack from a more advanced midfield role, Cancelo has ultimately proved to be an astute acquisition for the Premier League champions and earned a new five-year deal until 2027 back in February.
5. Ruben Dias - Benfica to Manchester City - £61.2m
Man City did not think twice about spending upwards of £60m on Ruben Dias back in 2020, with the Portugal international developing quite the reputation in the heart of the Benfica backline.
With three goals and five assists to boast from his opening 90 games in sky blue, as well as two Premier League crowns and an EFL Cup, Dias represents the third of three worthwhile expenses for Man City in the top 10 of this list.
Pep Guardiola has had no qualms about handing the towering centre-back the captain's armband over the past 12 months, and he warranted a new-and-improved contract until 2027 just one year after arriving in the Premier League.
4. Lucas Hernandez - Atletico Madrid to Bayern Munich - £72m
Still holding the record for the most expensive ever signing by a Bundesliga club, Lucas Hernandez was the new £72m man at the Allianz Arena in 2019 after ending his stint at Atletico Madrid.
As a World Cup winner with France, Hernandez slotted straight into the Bavarians' backline, whether that was in the heart of defence or at left-back, and he has already scooped European and Bundesliga titles with the Munich giants.
Legal problems have since overshadowed Hernandez's largely impressive exploits at the back for Bayern, as he successfully appealed a six-month prison sentence last year for violating a restraining order against his partner.
3. Virgil van Dijk - Southampton to Liverpool - £76.2m
At the time of Virgil van Dijk's £76.2m transfer from Southampton to Liverpool, the towering Dutchman boasted the tag of being the most expensive defender in world football - a label he has since lost.
However, Liverpool have not lost very often with the 31-year-old in peak condition, as Van Dijk continues to be mentioned in debates for the Premier League's greatest-ever centre-backs after winning no fewer than six different pieces of silverware with the Reds since 2018.
The centre-back's devastating ACL injury in 2020 has had very little impact on his ability to read the game and provide a constant threat at the other end of the pitch, and only Lionel Messi finished above him in the voting for the 2019 Ballon d'Or.
2. Matthijs de Ligt - Ajax to Juventus - £77m
Martinez's move to Man United will not be the first time that Ajax have secured a hefty fee for a young defender, as Matthijs de Ligt was snapped up by Juventus in the memorable summer of 2019 in Amsterdam.
The Netherlands international was still a teenager when he reached the semi-finals of the 2018-19 Champions League, and after becoming the Eredivisie champions' youngest-ever captain and winning the Golden Ball in 2018, De Ligt commanded a £77m fee to leave Ajax for Juventus.
While at the Allianz Stadium, De Ligt has won all three major domestic trophies Italy has to offer, but the 22-year-old is set to be the subject of yet another big-money move this summer.
After informing Juventus that he wishes to leave, De Ligt is now on the verge of a £67.8m switch to Bayern only a few weeks after it was reported that he would fetch a record £103m.
1. Harry Maguire - Leicester City to Manchester United - £80m
It only took Harry Maguire six months to wear the captain's armband for Man United after his record-shattering £80m switch from Leicester City, but it has not all been sunshine and rainbows for the 29-year-old.
Several Red Devils defenders were criticized for their showings last term, but Maguire was undoubtedly the most scrutinized of them all after a number of underwhelming displays could only help Man United finish sixth in the table.
However, the England international has already been handed a huge vote of confidence by Ten Hag, who has confirmed that Maguire will retain the captaincy as he seeks a fresh start under the wing of the Dutchman.