The 36-year-old central defender, regarded as one of Argentina's best ever, was to be unveiled on Wednesday having left Real Zaragoza as a free agent.
"They were the only club that really sat down with my agent and committed themselves to bringing me (back). Others showed interest but never offered anything concrete," Ayala told reporters.
His former club River Plate and Independiente were also interested in the defender, who helped Argentina reach the quarter-finals of the 1998 and 2006 World Cups.
"I'm here to help enhance Racing's history," Ayala said of a club that is one of Argentina's 'big five', but spent last year fighting to stave off relegation and whose only league title in more than 40 years came in 2001.
The move re-unites Ayala with coach Claudio Vivas, who was assistant to Marcelo Bielsa when in charge of the national team between 1999 and 2004.
Vivas was one of the more active coaches in the transfer market during the summer recess in January, signing sought-after Argentine strikers Claudio Bieler and Gabriel Hauche.
His team made a fine start to the Clausura championship last weekend beating Rosario Central 3-0 and were away to Colon in Santa Fe province on Tuesday night. Ayala could make his debut at home to Arsenal on Sunday.
Ayala, who began his career at Ferro Carril Oeste, joined Napoli from River Plate in 1995 and moved on to AC Milan in 1998. He left Italy for Spain in 2000, first to Valencia and then Zaragoza in 2007.
He won 115 Argentina caps and captained his country in a record 63 matches. He played in several Copa America tournaments, including the last two finals in 2004 and 2007 won by Brazil, but only won one international title, the Olympic gold medal in Athens under Bielsa in 2004.
Ayala was in Bielsa's squad at the 2002 World Cup but suffered a muscle injury warming up for their opening match against Nigeria in Japan and played no further part in the team's failure to qualify from their group.
