Frank de Boer's champions came out on top in the 176th edition of Der Klassieker to move 12 points clear of fourth-placed Feyenoord, dealing a potentially fatal blow to the Rotterdam club's slim title ambitions.
Graziano Pelle - who was allowed to play after having a one-match ban for his outburst at Twente last week suspended for 12 months - put the hosts ahead on the half-hour mark at De Kuip with his 19th Eredivisie goal of the season.
The sides went in level at the break, however, as Kolbeinn Sigthorsson equalised on the stroke of half-time.
And Ajax completed their comeback in the 72nd minute courtesy of Joel Veltman, as they extended their unbeaten league run to 14 matches.
The Amsterdam club's hopes of retaining the title were further boosted as second-placed Twente were held to a 1-1 draw at Utrecht.
Quincy Promes had put the visitors ahead in the 18th minute at the Stadion Galgenwaard, but Tommy Oar equalised almost immediately after half-time.
That result means Twente and third-placed Vitesse - who beat Roda JC on Saturday - are now both eight points adrift of Ajax with eight matches remaining.
RKC Waalwijk, meanwhile, remain embroiled in relegation trouble after falling to a 4-0 defeat to AZ at the AFAS Stadion.
Jeffrey Gouweleeuw and Nick Viergever had put the hosts into a two-goal lead by half-time, and that advantage was further stretched by Steven Berghuis' double.
Elsewhere, Elvis Manu's 77th-minute penalty rescued a 1-1 draw for Cambuur at Groningen.
