Jannik Sinner said it "means a lot to me" after he became the youngest man to complete the set of hard-court trophies with victory over Daniil Medvedev in the Indian Wells final.
World number two Sinner saw off the resurgent Russian 7-6 (8-6) 7-6 (7-4) in a match lasting just under two hours in baking conditions in California.
Sinner has won both hard court Grand Slams, triumphing at the Australian Open in 2024 and 2025 and at the US Open in 2024.
The 24-year-old has also lifted all six Masters 1,000 titles, external on hard courts - adding the Indian Wells title to victories in Miami, Toronto, Cincinnati, Shanghai and Paris - and the season-ending ATP Finals.
Only Novak Djokovic and Roger Federer have achieved the same feat.
"I knew that this was a tournament I haven't won, so I wanted to prepare in the best possible way, as professionally as possible," said Sinner
"Having this achievement now means a lot to me. Now I have couple of days to relax - there is not so much time in between here and Miami.
"It's again an important tournament in Miami, but we try to play the best tennis possible there too."
Andre Agassi won the five Masters 1,000 events that were held on hard courts in his career, as well as both majors and the ATP Finals.
Sinner, who had his Australian Open title defence ended by Djokovic in a thrilling five-set semi-final in January, also extends his head-to-head record against Medvedev, having won nine of their past 10 meetings.
The Italian did not face a break point against Medvedev and only lost four points behind his first serve.
He did not drop a set on his way to claiming his first title of the year and a 25th overall.
Despite the loss, Medvedev will re-enter the top 10 when the rankings are updated on Monday.
The former US Open champion almost missed the tournament, having been stuck in Dubai because of the ongoing conflict in the Middle East.
After arriving in the USA, Medvedev showed why he was previously at the top of the men's game, with a fine semi-final victory over world number one Carlos Alcaraz.
However, it was Sinner who edged the tight moments on Sunday, helped by his imperious serve.
Sinner won 43 of 47 first-serve points, hit 10 aces and won 60% of points behind his second serve to remain in control of the match.
Medvedev saved the only two break points of the final and came close to forcing a third set when he went 4-0 up in the tie-break.
However, Sinner reeled off seven points in a row to close out the match and win back-to-back Masters 1,000 titles without dropping a set.
Elsewhere, world number three Djokovic has withdrawn from this week's Miami Open with injury.
The Serb great has won the title six times and finished runner-up to Jakub Mensik last year.
However, the 24-time Grand Slam champion said a right shoulder injury had stopped him competing at the hard-court event, which begins on Monday.