The European global soccer body said in a statement that it would also allow teams to use a fourth substitute in extra-time during Champions League and Europa League knockout ties.
The new ruling, to come into effect next season, means that a player who been fielded by one club in the group stage of the Champions League, and is transferred during the January window, can play for his new club in the knockout stages.
The same applies to the Europa League.
Under current rules, clubs who qualify for the knockout stage of either the Champions League or Europa League are allowed to register three new players.
These can include one player who has represented a different club in the Europa League group stages but none who have been fielded for another club in the Champions League group stage.
The present ruling prevents Barcelona from fielding Philippe Coutinho in the Champions League this season as he has already played for Liverpool.
Alexis Sanchez was eligible for Manchester United, however, as he switched from Arsenal in the Europa League to the Champions League.
The new rules state that "that a club may register three new eligible players without any restrictions."
UEFA said the new ruling brought it into line with domestic leagues which "do not impose restrictions on the eligibility for competitions of players registered for a new club during the winter transfer window."
UEFA said the fourth substitution will be allowed following its approval by soccer's law-making body IFAB on March 3.
It said that 23 players, instead of 18, would be allowed on team sheets for the Champions League and Europa League finals, and the European Super Cup match.
(Writing by Brian Homewood in Berne, editing by Alan Baldwin)
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