The 33-year-old moved to Galatasaray last summer on a three-year contract as a free agent.
Babel made 20 appearances in all competitions for the Turkish champions in the first half of the season, scoring five goals but now returns to the Johan Cruijff Arena, where he began his professional career.
A statement on the club's website announced: 'Ajax has reached an agreement with Ryan Babel and Galatasaray SK.
'The attacker is immediately loaned from the Turkish club, where he is currently under contract. The agreement has a duration up to and including 30 June 2020.'
Babel left Ajax for the first time in 2007 for Liverpool but returned for a one-year stint in the 2012/2013 season.
The nomadic frontman has also appeared for Hoffenheim, Al-Ain, Kasimpasa, Besiktas, Deportivo La Coruna and Fulham in his playing career.
In a video posted on the club's Twitter feed, Babel said: 'Ajax friends, I'm back once again.
'I'm happy to be here, looking forward to wearing the shirt again, for the third time.
'Let's have some great moments together again, and thank you for your support.'
Ajax are three points clear at the top of the Dutch Eredivisie heading into the halfway stage of the season.
Babel has been a member of the Netherlands national team since 2005, featuring 63 times and scoring ten goals, and was part of their squads that finished as runners-up in both the 2010 FIFA World Cup in South Africa as well as the UEFA Nations League in Portugal last summer.
