The choice of the team cannot be underestimated: the Spaniard, who will be 39 years later this month, won his two champion titles with Renault in the 2005 and 2006 seasons.
The veteran driver made 314 Grand Prix starts during his F1 career, recording 32 victories and 97 podiums (his victories and podiums place him in sixth position on the all-time list) in three teams - Renault, McLaren and Ferrari - and his return will mark the first time that a driver has had three separate stints with a single F1 team (2003-06, 2008-09 and from 2021).
While Renault has not confirmed whether the deal would last more than a single season, the driver himself clearly has designs upon his return that are not so much a curtain, but a significant new chapter in his impressive career.
In addition, he believes that the new regulations - supposed to be introduced in 2021, but now delayed until 2022 due to the coronavirus pandemic - will give him and Renault a chance to be more competitive.
Veteran Alonso will replace Daniel Ricciardo who moves to McLaren - the same team the Spaniard left in 2018 - and will see him lead alongside one of the hottest young talents on the grid, Esteban Ocon, 23.
In the first race of the 2020 season in Austria, Ocon and Renault finished in 8th place.
"Obviously, it's a very happy day," said Amanda Davies, of CNN World Sport, 38. "When I left Formula 1, I felt that it could be a possibility to come back in 2021 ... I could breathe a little bit of Formula 1 because I have 18 consecutive Formula 1 seasons, which are quite demanding. I think I needed this downtime and I am coming back stronger now than ever. "
But it's not like he has spent his time away from grueling sport, feet at home, watching the next generation of F1 drivers, some of them almost half his age, in the hope to imitate Alonso's achievements.
Either way, Alonso's drive to succeed and his thirst to stay competitive remains intact.
"I hate to lose," he told CNN Sport with a big smile on his face. "I love to win. But, you know, I don't like to lose anything, and that would be difficult. Especially in 2021. We know where the car is now.
"We know that next year's performance is a bit of a carry-over from this year. So, I am aware that we will have to work a lot and we will have to slowly improve the car. But, you know, I am ready to take up this challenge also. "
However, when all is said and done, and business and professional considerations are put aside, to what extent did the romantic of Alonso make a decision when he returned? After all, he talked about Renault offering his best memories of F1.
"The romantic is an important factor in this type of decision," he said. "Coming back to Renault is a plus for me, because I know the people on the team, I know a lot of members who were on my time a few years ago. And also the commitment that
Renault made Formula 1 and the future of the team that the team has now, I think it was important. "
Judging by the sound, Alonso's future in motorsport still has many laps, races and - potentially - years to go. So how long does he plan to get behind the wheel?
"I think 10, 12 (years)" he begins, probably joking, but again, maybe not. "Let's see. When I was 25 or 28, I thought maybe three or four more years in Formula One would be enough.
"But now, at 38, for whatever reason, I'm developing more skills. I was driving in different cars, different categories, learning different driving styles. I said it before, I feel cool now , ready to travel, ready to do different things, to work on the simulator.
"So I feel better now than when I was 25 years old. So I can't say now that there are only two or three more years in me. You know, maybe he there's more."
