Teams will be able to use a fourth substitute in extra time in next season's Champions League and Europa League, Uefa has announced.
Clubs can also list 12 substitutes on teamsheets, rather than the current seven, in the finals.
Other rule amendments from European football's governing body include allowing coaching staff to use hand-held electronic devices.
They can use equipment including mobile phones, tablets and laptops.
The changes take effect immediately and also apply to other competitions, including the 2020 European Championship.
A fifth substitute will be allowed in next season's Uefa Youth League, plus the men's and women's European Under-17 and Under-19 Championships.
Uefa says the hand-held electronic equipment can be used by coaches in the stands or on the bench if it directly relates to player welfare or safety, or for tactical/coaching reasons.
It means medical staff will be able to review potential concussions on laptops.
Officials and medics did not see a collision between Loris Karius and Real Madrid's Sergio Ramos in the 2017-18 Champions League final, which may have led to the Liverpool keeper's concussion.
In 2016, rugby union's English Premiership became the first league in the world to allow access to tablets in order for staff to review potentially concerning incidents.
Real Madrid's Alvaro Morata was the sport's first fourth substitute at the Club World Cup last year as part of a trial.
Fifa, the sport's world governing body, approved the use of a fourth substitute in extra time earlier this year and the rule has applied at the World Cup in Russia.
The Football Association announced last summer that it would permit an extra substitute in the latter stages of the FA Cup, from the quarter-finals on, while the Football League has permitted a fourth substitute in extra time in all EFL Cup ties since last summer.
Manchester City were the first team to make use of it in England, as Kelechi Iheanacho replaced Raheem Sterling on 105 minutes in their semi-final defeat by Arsenal.
In Scotland, Ayr United benefited from the rule change in their Scottish Cup fifth-round win over Queen's Park.