Five substitutes will be allowed in the Champions League and Europa League from the group phase, UEFA announced on Thursday.
The executive committee of European football's governing body approved the regulation change for this season, which will apply to other UEFA competitions such as the Nations League.
A proposal from FIFA to increase the number of substitutes to help clubs manage the workload created by the coronavirus pandemic was made to the game's law-making body the International Football Association Board earlier this year.
IFAB approved it, and after initially applying it to the 2019-20 season, the temporary dispensation was extended into 2020-21.
The Premier League adopted five substitutes during Project Restart over the summer but voted to revert to three for the new campaign, a decision which Manchester City manager Pep Guardiola criticised.
He said it was an important way of protecting players, and world players' union FIFPRO welcomed UEFA's decision on Thursday.
"This is the right decision and a good first step to ease pressure on elite-level players," its general secretary Jonas Baer-Hoffmann said.
"We will continue to push for further innovative ways to reduce their mounting workload and protect their health and performance."
UEFA's ExCo was meeting in Budapest, where the Super Cup between Bayern Munich and Sevilla was being staged on Thursday with a limited number of fans in attendance.
UEFA president Aleksander Ceferin defended the decision to allow spectators in, saying: "We will have 16,000 spectators in the stadium, not even 30 per cent. The measures are strict and there is not a question of health risks for the fans.
"We will of course not be dragged into any political discussion about that. The easiest thing for us would be not to do anything. If you don't do anything, you don't make any mistakes but of course nothing moves."
He said UEFA and the Hungarian government would analyse the pilot event before any decisions on any wider moves to allow fans back into UEFA competition matches, which he pointed out would need approval at national government level in any case.
"Fans and players are the essential part of football," the Slovenian added.
"We will take care of health, we are working day and night on it but from today on we will see what we decide, we haven't decided yet anything about spectators in the future at the European matches."
UEFA also decided to make next year's March and September international breaks triple-headers, with FIFA having already granted an extension to the international windows of October and November this year to fit in an additional match per country.
