Here, we take a look at the transfer rules which govern each league in the winter window.
Clubs are still able to sign players as free agents after the deadline, as long as they are not officially attached to any club when the window closes, which is Jan. 31 in most leagues.
Premier League transfer deadline
The Premier League led the way in Europe last season in bringing forward the transfer deadline to before the first match of the season, but it matches its continental cousins when it comes to the January window.
The transfer market will close at 11 p.m. GMT / 6 p.m. ET on Friday, Jan. 31.
The deadline is the same for Championship clubs, should they be looking to bring players in on loan from the Premier League.
The Premier League will decide when its summer window will close at its upcoming stakeholders meeting, on Feb. 6. It has been suggested that the window will only close for domestic transfers before the season begins, with clubs able to buy from other countries (as well as sell) through to Aug. 31.
La Liga transfer deadline
The window will close at 11 p.m. GMT / 6 p.m. ET on Friday, Jan. 31.
Serie A transfer deadline
Last season, Italy was also due to have a truncated winter window, closing on Jan. 18, but a few weeks before clubs voted to return it to a month-long window.
The January window will close at 7 p.m. GMT / 2 p.m. ET on Jan. 31.
Bundesliga transfer deadline
German clubs have set their transfer window at 5 p.m. GMT / midday ET on Friday, Jan. 31.
Ligue 1 transfer deadline
Ligue 1 will close its window on Friday, Jan. 31 at 11 p.m. GMT / 6 p.m. ET.
MLS transfer deadline
Major League Soccer's first window of the season official opens on Wednesday, Feb. 12, though many transfer deals are already being announced ahead of the start of the 2020 MLS season on Feb. 29.
The first window of the season will close on Tuesday, May 5.
A handful of top leagues will still be able to buy players after Jan. 31. The deadlines are:
Jan. 31: Australia, Mexico
Feb. 2: Portugal
Feb. 21: Russia (window opens Jan. 22)
Feb. 28: China